I see your point and it can become an issue when members of your group constantly go AFK / BIO / BRB etc.
The best way to manage this is at the start of the group notify members what the aim is. On a short raid/instance there is no need, treat it like a car journey ..... go for a / or a dump prior to the start whilst buffing then there "should" be no need to stop during. On longer raids you are just going to have to accept the fact that we don't all have colostomy bags and most of us don't wear nappies hence need to go afk to loaf or it would get very messy.
Basically if you are leading the raid / instance you can control these by natural management.
you see my point?