Not sure if I understood the question correctly, but as a suggestion, could you create a separate registration type for those "attendees" that are ultimately accepted and then setup their reg settings where reg fee is $0 for that reg type. Once accepted, you would switch reg types from what you have as a default. Additionally, those that are not accepted would fall under a "vendor" reg type where a reg fee would be listed. Hope this helps!
------------------------------
Jenn Wang
Global Digital Events Advisor
EY Global Services Limited
------------------------------