How about havign each mosnter with a set number of tentacles from the start? It would take a full action to use them all, but in exchange they'd get reach (2-3 squares) and and have them initiate grapple on a successful hit? Let grapppling/pinning use up one tentacle, while others will incraese escape dc and still count towards gaining pleasure (one can hold her in place while the rest do their work). Put a limit on how many targets a monster can grapple at a time (maybe up to 2?). Let tentacles have 1hd  on their own, and I like the idea that cutting them will do at least some damage to the mosnters. Give every monster a power to regrow all destroyed tentacles as a swift action every 1d4 rounds. That way, reducing the number of tentacles will still buy us some time, and we could have multiple enemies out there, preventing us from ganging up on one?