I figure however you do it the solution will be fairly technically involved. I think that it would be possible to do it using postfix as the MTA and setting up a virtual user mapping using your MySQL table as the backend.
On a non-technical note I'd be careful about supplying users with mail redirects, and what you promise them in terms of service. An email address and the ability to receive mails is usually pretty important to people, if the forward is down and they're unable to receive mail they'll probably get upset. Personally I would never go down the route of giving users mail forwarded addresses because of the possible support issues involved.
...just my 2c
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