Track Additional Data With User Accounts

I was asked to develop a site for an investment firm. They wanted to allow people to sign up, have someone at the firm approve the membership, and then allow approved members to see a specific custom post type (all of the companies that the firm is currently funding). S2Member allowed me do this easily enough.

Of course, then the client throws in another “small change” at the last minute – they want the ability to show which companies a user account has invested in. The users cannot invest directly from the site, but the site admins want the ability to log in, click on a user account, and enter in some data as to which companies that person has invested in.

I didn’t design the site with this in mind, and I’m kind of stuck at this point. I would have gone with a more “estore” type plugin if I had known they wanted to associate who has invested in what. I could make this a simple text field, but that’s pretty cruddy, and terribly error prone. What’s killing me is that I do have a list of available companies. It would be trivial to produce a list of these companies in a standard form (by querying for the custom post type) and allow someone to check them off. But I’m really trying to work within the confines of WP and S2Member.

Am I barking up the wrong tree entirely? Should I just write a custom admin panel for the investment tracking? Or is there some kind of way to track an “investment history”/”purchase history” with a user account?