It depends on the material for how the radio may be effected, but I do think all MR's are plenum rated, basically meaning they can be installed in the ceiling without causing a fire hazard.
For safety, I would be sure to mount it securely so it can't fall on someone's head if they open a ceiling tile, for example.
I once had to install ~200 APs of another manufacturer above the ceiling grid for "architectural reasons" (they were ugly). That reminds me of another suggestion- label/mark the ceiling where the AP's are placed so you can find them again 😉
- Ex community all-star (⌐⊙_⊙)