Write up a simple app, sell them the right to sell it, but retain rights to release it under the GPL.

Kewl!

but you'll probably run into problems if you try to do this with a VBA macro (IANAL, but you know how Micros~1 feels about that "viral" GPL, now don't you...?)

so get a copy of Borland's Delphi or C++ builder (the Professional version, that contains the Interbase database engine), or, maybe better, Kylix (so you can distribute a Windoze and Linix version), and develop it there.

Again, IANAL....