Externally Venting the Crankcase-to-Oil-Tank Breather Hose
NOTE:  In as much as I would like to say that this modification is good for a couple of horsepower - I doubt it.   I also don't recommend this approach, since this line will spit oil when the engine is run hard, and that oil will make a mess of the side of the engine, and likely dampen the drive belt and the rear tire.  I tried this - but later ran a tube to a catch can that vents to a breather in the recess under the passenger seat area which works far better!  The approach described below is old info, and while it may look trick, it can cause problems with spitting oil on the rear tire and I would not recommend it.
Just for historic purposes, I've kept the following notes....
After running like this for 2,000 miles, it is apparent that when the engine is run under high rpm, the crankcase breather tube will vent significant pressure and oil.
Also, when this tube is connected in its stock configuration to the oil tank, when the engine is revved, the oil pump will scavenge very efficiently.  That means that anyone who fills the XR's oil above the middle of the dipstick (checked hot after running the engine) is very likely recirculating oil to the crankcase through this vent tube while riding the bike - and is very likely blowing excess oil out of the head breathers and breather port in the top of the newer XR oil tanks.
First, here is a look at the existing vent hose.  A 5/16" segmented line connects the oil tank to the crankcase breather port.  It leaves the crankcase through a 1/8-27 to 5/16" barbed hose fitting.  A 9" piece of 5/16" flexible rubber hose connects to a piece of hard plastic tubing that runs under the oil tank and up the back side of the tank inside the rear wheel splash guard.  The hard tube then connects back to a 4" piece of 5/16" rubber hose that connects to a fitting on the back of the oil tank.  (Later I'll talk about how the hard plastic tube can be removed and pointed upwards and to the rear of the bike to run to a catch can.
Here is the crankcase breather line.  The blue pointer points to the fitting that exits the crankcase.

Vent Hose from Crankcase to Oil Tank
In the picture below, the blue pointer is pointing toward the hard plastic tube from the oil tank.  This hard tube plugs in to a soft rubber hose connected to the underneath of the top of the oil tank at the back.  The picture shows a 5/16" rubber cap that I put on the line temporarily to prevent oil from splashing down on the engine - but unless you are running a newer vented oil tank (with the 10 psi pressure relief valve) and never overfill your oil, you definitely want to reroute this tube up and to the rear of the bike to let excess oil and pressure bleed off. 
The hard tube plugs in to a short soft rubber stub.  I simply unplugged the hard tube, pulled it out, rotated it 180 degrees and reinstalled.  This pointed the hard tube to the rear and upward, instead of in its normal position to the front and down.  I then connected a clear rubber hose to the hard tube and ran it up and back to the rear of the bike and to a catch can.  I put clamps on the  tube connections so that they would not leak oil pushed through the tube.  Click here to see the oil catch can installation.
Now if oil blows out of the tank, it will blow out the back of the bike near the end of the seat cowl instead of dropping on the engine, drive belt and rear wheel.  The oil temporarily fills the catch can, and then when the engine pressure drops, it allows the oil to gravity feed back to the XR's oil tank.  This lets me fill the XR1200's oil tank to the top of the dip stick without venting excess oil out of the breathers when I am running the engine hard.
Even with the oil tank line run up and to a catch can, the engine case breather port will still vent air and oil when the bike is run hard.  It will blow excess oil on the side of the engine, drive belt, and rear tire if run like shown below.  I can say that if you are going  to externally vent the crankcace - it is important to vent the line from the oil tank up beyond the rear wheel and to a catch can.  This breather filter idea looks trick but is not a good idea to do.

Connection at Oil Tank End

Here is the the 5/16" rubber hose and the pipe fitting that connects it to the crankcase.  I merely unplugged the hose from the hard plastic vent tube and unscrewed the fitting from the crankcase.

5/16 Hose Connection at Crankcase
The below picture has another view of the vent hose from the crankcase.  I am holding the hose at the end where it plugs into the hard plastic vent line.

Another View of Connection at Crankcase

The below picture shows the one-way air valve (Hayden Enterprises from DragSpecialties DS-289525) and small K&N filter (62-1320) which I did run for a while on the crankcase breather port.  I replaced the 5/16" barbed hose pipe fitting on the crankcase with a 3/8" fitting, and ran 3/8" rubber hose from the crankcase to the air valve.   The air valve just plugs into the K&N filter.  This looks neat, but I later re-routed this line up and to the rear of the rear tire.

Air Valve and Filter
While I do think that routing the engine case and oil tank has some merit in terms of decreasing engine case pressure and catching scavenged oil which surges to the oil tank when the bike is run hard - again - I do not recommend putting a filter on the line in front of the rear wheel like shown above - instead the line should be routed to a catch can and vented behind the rear wheel.
If the oil tank overflow line is routed externally, I recommend hooking it up to a catch can like shown in the following link.

Oil Overflow Catch Bottle - Oil Tank Breather

After doing this I checked the air flow out of the rocker cover breather lines and the new crankcase breather line.  Under normal conditions the air pushed by the bottom of the engine escapes fine from the rocker cover breather hoses, and does not exit the crankcase breather hose.  When running the engine hard though, significant air and some oil escapes from this engine case breather port.   
Re-routing the oil overflow from the oil tank to a catch can does  prevent a potential problem with excess oil draining back from the oil tank to the bottom of the engine, which could contribute to wet sumping and possibly cause breathing issues out of the top of the engine because oil can not drain back to the bottom of the engine properly.  And, it is clear that filling the oil to the "full" mark on the dipstick (checked hot on the side stand after running the bike) will cause the bike to be overfilled during operation - which this mod addresses.  However, if the engine case is vented externally the tube should be run to a catch bottle and vented behind the rear tire - and not to an air filter in front of the rear tire like shown in the pictures above. 
NOTE:  If you simply plug the vent tube on the oil tank - on an older oil tank without the pressure relief port, this could contribute to weeping or leaking from the oil tank; on a newer oil tank with the pressure relief port this could cause excess oil to blow out through top of the pressure relief port on the top of the oil tank and drain down on to the engine and rear wheel.
Click on the below link to go back to

The XR1200 Engine (unplugged) - Overview

This is another view of the K&N breather filter and Krank Vent.  Though it is a little hard to see, to re-route the larger 3/8" breather hose through the path of the stock 5/16" hose, I had to trim the corner off the top of the transmission pulley cover where it meets the cam cover.  I cut a small triangle off the pulley cover, which aligned the inside edge of the cam cover and pulley cover and let the larger hose run straight.

Air Valve and Fillter alt view