This is something that is not supported in ADFS V1. You can do this with WIF and I believe there was even a lab from PDC that showed you how to build an STS using the ASP.NET SQL membership system as a backend. I'm not sure if ADFS V2 (the full server) lets you sub in a different auth store directly or not.
There is some trickery you can do with shadow users in AD and protocol transition logon, but it doesn't sound like a good fit for what you client wants to do.
Even though ADFS V2 hasn't shipped yet, WIF was released back in November.