Talk to your dentist.
If you tape your mouth shut, it would seem that the pressure in your mouth should be the same as the pressure in your nose. Wouldn't the sinuses be at the same pressure as your nose, so if the pressure in your nose is the same as the pressure in your mouth, there's no pressure difference? Wouldn't this reduce any risk of blowing a hole from the tooth socket to your sinus?
A CPAP pressure of 20 cmH2O is a rather high CPAP pressure, but it's only 0.3 PSI. It would seem that just normal breathing, drinking through a straw, talking, coughing, sneezing, whatever might well be higher.
If you were drinking water from an 8 inch long straw, that will give you 20 cmH20 of vacuum. It really doesn't seem like that much pressure. Or if you had a mouthfull of water, put an 8 inch straw in your mouth and blew the water straight up into the air, just dribbling out the end, that's 20 cmH2O as well.
Or for that matter, put on your CPAP machine. Purse your lips and let a small stream of air blow out your lips. That's how much pressure. Or simply keep your lips together and feel how much pressure there is on the inside of your mouth. Move your lips like you're making a "Pa" sound while using your CPAP and feel how much pressure there is. Then make the "Pa" sound while not wearing the CPAP and talking normally. I suspect you'll be making as much pressure while talking as while CPAPing. Close your mouth and breathe sharply out your nose. Feel the pressure of that vs. CPAP pressure.
Like I say, consult with your dentist, but I don't think CPAP pressure is going to be high enough to make more sinus damage possibility than normal daily activities such as eating, drinking, talking, breathing, etc.