It should probably be in the genitive (Sabinarum rapiendarum), and fuisse isn’t needed. It’s the same sort of structure that GJCaesar and I have described (in slightly different ways!) on the spolia opima thread.
Even though in English we would say “the signal for snatching the Sabine women”, I’m pretty sure it would be a genitive in Latin, partly because the dative would certainly be used for the people to whom the signal was given (rather than the action for which a signal is required).