The auth provider only does one thing: determine through credentials your identity and to create a sharepoint user  token on that identity
To get metadata off a user the information has to come from somewhere. Using the BDC to mine that data is a very good  start.

You'll need to implement IdEnumerator and SpecificFinder methods. It should be pretty easy. When you implement the SpecificFinder, just expose all your fields in the Membership table (forget the exact table name). When you implement the IdEnumerator, you just need to expose your identity column usually a unique index field.

BDCMetaman can do both of these if you wish. You'll just need to connect to your SQL datasource as either PassThrough or RevertToSelf authentication.

After you have the ADF setup, you'll need to connect up the BDC source for user profile crawl. Then you'll need to update the profile properties and make sure they map to the properties you exposed through the BDC. Again this process is pretty straightforward.

I don't think 90K user profiles  is an issue, it could take a few hours to crawl, index, and populate the user store though.

As a test, you might start with only a few users and see if that works. You can do this by modifying the SQL statement in the SpecificFinder method by adding some type of limiter like a where clause.