The theory is that you'd have a tentacle placed just inside the mouth, at its base, and so it would be poking out of there, if entered from butt. Or vice versa. So you know how the existing tentacles just float in place and at the right angle you can see that they lack a mesh to cover up the base? Do it something like that, but with the tentacle starting just inside the mouth or butt for the exit point, and have a normal tentacle entry using existing assets for entry.
Course it would need bulges, and that might be the more complicated part.