I'm no DoDMERB expert but I doubt that a bite guard itself would be high on the DQ list. What's important is what you're being treated for. Bite guards are used for many things... grinding of teeth, snoring, and I'm sure many others. What puzzles me is why the condition is (apparently) temporary. It's not clear from your post what you're being treated for, but there is certainly no reason you should disclose it here.
I believe your appointment materials says you need to report any health changes to DoDMERB. I would recommend doing that because you don't want it to be discovered on I Day and turned back. If you do report it, be sure you've got the condition/diagnosis straight. I would also accompany it with whatever records your doc thinks would be useful, especially the recovery prognosis. You don't want to add any unnecessary confusion to the matter.
YMMV. Good luck!