1. s/specialized/separate/
  2. Use SASL, or another authentication protocol to talk to your auth DB.
  3. Profit!

If you're trying to solve a fundamental problem like authentication, odds are someone else has already spent a lot of time thinking up a good solution.

Re: LDAP

This may not be as good as a specialized solution, but it's probably a step up from throwing all your data into the same MySQL DB. ;-)

It's not particularly hard, either, just different from what you may be used to.