The further I got in this, the more I realized the issue is in figuring out the logic of cart.php and tweaking it to evaluate an extra field and then allow a zero-inventory product to be added to the cart.
I've burned many hours trying and am temporarily giving up on being able to open the store until this is done (Grrrr). So... I posted in "requests for services" if anyone wants to take a stab at this:
http://forum.x-cart.com/showthread.php?p=192468#post192468
-Michael