Vanilla is such a context dependant word, I think. I think some stuff in this game is already pretty kinky (which I like), but I guess everyone judges stuff differently. I definitely like the idea of this somehow being Asamis idea. Seems like you are also thinking along those lines. It would also be kinda nice if Sayuri would be into something the others aren't. Not sure what that should be though.
*edit*
One possible way into this is Sayuri showing unhappyness about her love situation to Asami (maybe even show jealousy). And Asami, realizing she is already sharing MC anyway, suggesting the hookup out of love for her mother. This would make the route more guild free and make Asamis personality seem more kinky. Some discussions about this between Asami and Yasuka/Noriko would make sense obviously.