Alternative to the Regcode module?
I think you'll need to do some custom magic voodoo ;)

I built a similar site earlier this year:   The client provided a list of users (with their email addresses) in an Excel spreadsheet.  I created a new column with a unique 8 character registration code for each user.  I then imported that file, including the code, as new users.

The registration code was saved as a plain-text field in their user profile.  

Create a custom user login form that only requires the registration code,  mail out the reg codes.