Take this with a grain of salt...but I think.........

You may have to write your own Custom Membership Provider ...... which can
simply be a wrapper ( Adapter Design Pattern ) for the
SqlMembershipProvider.

You'll need a "local" version..... which passes calls thru to WCF.

Then you'll need the ServiceLayer.<<I do this just to keep a clean
implementation

Then you'll need the server side code...with is the actual wrapper for the
SqlMembershipProvider.

..........

I think the trick is that you have to write a service contract...
IMyService....which implements MembershipProvider (or one of the higher
interfaces for this class).

Take a look at this example
http://sholliday.spaces.live.com/blog/cns!A68482B9628A842A!158.entry

ZebraControllerServiceWrapper
ZebraControllerFascade
ZebraController

one of the things you'll notice is that all of these implement
IZebraController

to get to your situation.... I ~think~

IZebraController : MembershipProvider

But wait..MembershipProvider is an abstract class...so I don't even think
that will work.

Maybe IZebraController can implement MembershipBase (?? going from memory
here)...or something like that.

Its an interesting problem.

Do a followup and see if that gets you anywhere, or if you hit a roadblock.