As registration is run through the Mambo or CB registration process depending upon what you've chosen to use, this would really be more a matter of hacking the registration process.

For mambo, it would be hacking the main code.  With CB, it's a matter of modifying the CB component to include such functionality.

Not sure about the mambo registration, but I know that over on the CB forum several people have made mention of this possibility, though little progress has been made as of yet.

If you or anyone else is interested, just hop on over and join in!