I think the bunny girl fetish isn't as big in Japan anymore? It used to be it was in anime everywhere, including Dragon Ball, but the last one I remember making use of it was Suzumiya Haruhi, although I'm sure others will come up wity newer examples. Anyway, yeah, Japan doesn't care about Easter. Martha might be a good fit for the event, as she's the Servant who would latch onto it the most, but it's almost as unlikely as a Thanksgiving event.
Either way, GO already has a Bunnygirl Servant in Riyo's Rider, and she was overlooked in favor of Riyo's Berserker, so it doesn't look like the game developers are that keen on the particular fetish.