There are many cultures and religions that have ritual observances at particular times of the day. If these people are not proving to be a safety hazard or causing undue inconvenience what is your problem? If you are ignorant of these practices, you should learn.
No it would not be "prudent" to make an announcenment, nor would it be good manners. Make them do it in the toilet?
Disallow them from doing "it" on flights. Why, who are they harming?
Anyone else seen this? Probably most people with any worldly experience.