Restricting MOP on a presale

Before I tax my brain cells trying to figure out if there is a way to do this, is anyone doing presales restricted to a specific method of payment online?  As in only for American Express cardholders?  I'd like to make sure they only use a certain MOP with this promo code, price type, allocation code or anything else I can do control it without modifying the API. 

Thanks,

Susan Crockett

CIO, Ruth Eckerd Hall