FAA goverened aircraft
In commercial aircraft there are o2 masks in the lav. You are expected to be in your seat during turbulence, a rapid decompression is an emergency situation, you may be covered in lav juice but you need to be provided o2.
As far as the rapid descent goes, you are live cargo, the airline takes steps to keep you alive. Making an announcement would be useless over the environmental sound not to mention a waste of their valuable time, even if you could hear it it would likely result in anxiety resulting in irrational actions. The submissive human response to self preserve is what the CAA's logically want (secure your seat belt, breathe air and ensure children next to you do so)
To add... Most modern aircraft have vacume waste systems, worst case scenario you may have your own brown trout and urine floating about the lav during a rapid descent