for the PIR you mean a temperature above 38/40°C on stage could be a problem?
-Very likely, they are looking for heat changes to surroundings. So, if your surroundings are 38-40°C your actor should have fever to be recognized by PIR. In theory, in practise is best to just try it with a cheap PIR-lamp (the one you can attach outside your house).
-With cams you can shoot into the direction you have no light fixtures: from back to front. Put cams into an angle that they are not getting any floor (unwanted shadows/lihgts), but the dark audience only. So, you have only lightened actors or dark background - but no unlightened actors!
Thats a way it could be done, depending on stage and light design.
May be you have to cope some regions in this video image by hand (or black quads), where there are small lights for emergency exits. But it shouldnt be huge areas.
Ultrasound is best choice, in fact, but video is the easiest in practise ;)