And what about this idea:
Having the sponsors being able to register before the non-sponsors, like 1 day before?
That would bring you a LOT of sponsors more (as many would be a sponsor just to be sure to be able to join), and would relieve a part of the burden of the inscriptions... no?
They already do that for Sci-fi conventions, allowing "Diamond Pass" holders to register before the "regular attendants"...
(to give you an idea of what a Diamond Pass is: in this case it cost at least twice the amount of money from a regular attendee of the con, but with free signings from all the actors included, and sometimes even a gala dinner)