I do not believe you will be able to modify the Java client to the extent that you are talking about in the current version of WinGate. However, I do know that what you want to do will be very easy in the next release of WinGate (7.0).
In the next release, you will be able to integrate WinGate with an ODBC database and create your own website to provide authentication to WinGate and manage access. The beta of WinGate 7.0 should be released before christmas this year, so you might want to participate in it to find out if the next version of WinGate will be aligned to your needs.
You can find out more information about WinGate 7.0 and sign up for the public beta at the following URL.
http://www.wingate.com/wingate-2007.php