Ok. I was making this way too complicated.
The solution I've settled on for now is to have a tentaclevaginal, tentacleanal, and tentacleoral script separate. They can be used at the same time since they don't require a giver part. It's a lot more versatile this way as well.
If anyone wants the scripts, the links above still work. Just add them to files/scripts/actions, and when you use a Scylla or another character you've modded to have 'tentacles' in the 'tail' slot, then they can get up to some good old hentai-style antics.
Enjoy!
(but... do let me know if someone comes up with something better.)