Dynamic role and memberhsip provider

 Hi all,
 
I'm wondering if it's possible and if so how to impliment it.
We write assessments for colleges, schools and training providers in the UK and around the world. We're developing a system based on ASP.NET roles and membership, we're going to be hosting the solution on one of our dedicated servers. However we're wanting every client to have their own database using URL rewriting to distinguish each company/organisation.
How can i change the role provider datasource dynamically e.g. in the url rewrite module or page initialise?
 
Regards
Liam

...