Used to be in CAO 99 or there abouts, which is now removed, and I think it is now up to operators who carry persons in custody to have a section in their ops manual to cover it. The body you are carrying the persons for would also have a say.
The old CAO used to have a limit on amount of prisoners that could be carried, 5 I think, and state that they had to be restrained (but not to the aircraft), could not sit near exits, and there had to be one guard assigned for every prisoner.
If you could find an old copy of the CAO's, circa 1997, you might find the old CAO.