2003 cr250 leaking trans fluid from fill plug

Are you putting the correct amount in and torquing to (or close to) spec? I would buy a new plug and O-ring to try. The mating surface could be messed up.

Almost seems like the transmission cavity is becoming slightly pressurized due to a bad seal somewhere.

Does the old rubber glove over the uncapped fill port with the bike running trick to see if theres any kind of vacuum or pressure build up occuring apply to 2 stroke motorcycles the same way it does with PCV valve equipped automobile engines?

I have the same bike and can help you out if you find something you suspect ....IE the glove inflates on your end etc. to see if I get the same result. I've never had that issue so let me know if there's anything you'd like me to check or try if it will help you out at all.

Check to make sure your crankcase breather hole and hose isn't clogged, pinched or obstructed. If its free and clear and the oil isnt over filled, I'd be hard pressed to believe there is enough pressure to blow it out past even an ill fitting o ring on the filler cap.