I recently discovered that after a year of trying I can keep my tongue on the roof of my mouth - but only if I wear a chin strap. No apparent need to glue the lips closed with denture adhesive or use tape (hard with a beard!), but I need to continue using the strap. It's great when you discover you can do it - makes preparation to sleep one step easier! Of course, with a full face mask you wouldn't have to bother with any of that, but it would bring its own set of problems!
Oh, and better to have your tongue on the roof of your mouth than planted in your cheek!
Diagnosed September 2009, Respironics System One w/ C-flex and humidifier
Aloha nasal pillows
Not a medical professional, just a patient with severe OSA (31) who has done a lot of reading.