First a leaking transmission PTO does not cause transmission failure. ONLY FAILURE to seal or to routinely check fluid levels causes transmission failure! thus end user is the cause!
There is no way to check fluid level when the pto is covering the fill and inspection holes. At least that's how mine is set up. The seal leaked on the actuator arm and over time I lost a syncro. It's guess work. User is the cause; sure. What do want to do, pull the pto,drain the trans, mount the pto back up, then fill it? C'mon over and be my guest!
If leaking and you cannot access the fill port, then this poses a problem but not one that is impossible to check, just more complex and well worth the effort based on loss.
Get a clean oil collection pan, drain oil measure and amount in the pan, compare to what is required, install plug and torque, remove shifter and pour in required amount and if a leaker add 1qt more for good measure.
Mark down when you did this, and check again when needed...based on leakage..
Maintenance is a requirement and people come up with tons of excuses for not doing it and blame a leak for failure when in fact it was lack of maintenance.
Sorry for the hard line approach; however I take this really serious and attempt to find solutions not excuses after loosing an engine because of my own fault.