I have just stumbled over Skritter and I really love the concept, but unfortunately I am one of those damn people clamoring for Cantonese support. I know you don't have any plans as per the FAQ, but there really is nothing comparable out there for Cantonese - and us Cantonese learners are pretty much stumped.
However, I think there would be a very simple solution for this - why can't users simply add custom romanization / translation? Since you already support traditional characters, all that would be necessary is for the user to enter the Chinese character (so the system recognizes the correct writing pattern to follow) and a custom romanization / translation. This seems like fairly simple functionality and would tie in very simply with what's already in place.
Look I really love Skritter and I would give you guys all my money, and there's a lot of Canto students who would love something like this. Thoughts?