I am not sure what your implementation timeframe is, but this is the exact scenario that we envisioned with the new authentication refactoring that will be released as part of 4.6.  The system lays out a simple framework that would allow you to integrate pretty much any form of authentication and still have it link up with either an existing DNN account or allow the user to create a new account.  Take OpenID as an example.  In the new system I can enter my OpenID into the login box (assuming OpenID auth has been enabled in DNN), and the system will automatically redirect the user to the OpenID provider to authenticate the user.  Once authentication is complete, then the user is redirected back to the DNN site where they can link the OpenID account to an existing or new DNN account.  Once linked, the OpenID auth just logs the user into the system without needing to link the account again.

Right now we anticipate that 4.6 will be publicly released prior to OpenForce Europe in mid-September.