Hahaha. Well, I gues it should work cuz the ROM is replacing all the system files so your core is Froyo. but your data is 2.1
As far as Maps is concerned, there is definitely NO GUARANTEE that anything will work if you have 2.1 data with a 2.2 /system. And how the market would respond to it...is anybody's guess.
I guess I would say go for it at your own risk lol.