Seeing as i carry an NSR motor where ever i go, i decided to see what has to happen for the gearbox to get pressurized from the crankcase. There is a crankcase join between them, which is highly unlikely to fail. In this pic you can see the crankcase breather occupies the space between the reed valve mounts for cylinders 1 and 2, an example of economy of design. This engine has a hose fitted to the gearbox vent, but i don't know where previous owner had routed it to.
In this pic you can see on the left hand side of the engine, there appears to be no common boundaries between the crackcase and the gearbox.
On the right hand side however the crankcase and the gearbox share a boundary, there must be a seal around the crank, as the crank protrudes into the gearbox, so it appears the only way the crankcase could pressurize the gearbox is if this seal fails.
As remote as this sounds it is in my humble opinion that it should be vented to somewhere, if it was a race bike it would be vented to a transparent catch bottle visible to the rider.
Cheers