I have wondered at times about stopping the drain tube above the oil level but have never thought about pressure in the bell housing. Everything I have read says it would make thgings worse but we've all heard about Fake News!! There would not be much pressure build up in the bell housing (has it ever been measured?) due to leakage and what pressure is developed is only by chance. That is, nothing is designed to build up pressure in there. If it were, I suspect it would also slow or stop oil leaking from the gearbox from around the selector shafts.
A very interesting theory. Now, how to pressurise the bell housing and test it.
Calling for volunteers.
