I currently run a heavily-modded Snitz-based community (42 added mods, plus custome code) and am considering migrating it to vBulletin. However, that decision isn't easily made because I can't get access to view the hacks and mods available to vBulletin.
I completely understand that you want to prevent downloading unless the person has a VB license, but IMO you're doing people a disservice not to allow people to research what's available. Sure, you have a forum for pre-sales questions, but you certainly limit people from performing their own research...
Have you considered changing some of these restrictions?