To be honest, I wouldn't recommend implementing custom membership at this time. It would probably be better to wait until after Telerik supports doing so. Looking at how they've baked certain things into their membership (etc.) classes, I have some doubts as to their ability to provide a nice, smooth way for people to implement customer membership--unless they make some design changes. But I could be wrong.
In any case, some of the things I did:
I subclassed MembershipDataProvider -- and had this new class implement ICommonDataProvider.
I also had to create my own User subclass and use that for my users.
And as I said in another thread, unless I actually cache these users, I get logged out of the backend very quickly -- within seconds. For whatever reason, unless the actual instance (as opposed to an equivalent instance) of the logged on user is in the collection of users, the person is logged out.
There seem to be a lot of holes and you have to jump through a lot of what should be unnecessary hoops to get things to work (and ultimately, my stuff doesn't currently work) that I would suggest not pursuing it right now.
But by all means, if you do get things working, let us know.