One would think that given the volume your dealer does with the local Enterprise branch their discounts would apply on most/all models in Enterprise's fleet.
Given the dealer's strategy to pay for loaners rather than hold 5 or 10 BMW's in inventory, I think they'd occassionally pay for a like kind Enterprise car if you pushed a little. After all; you ARE the customer.
Granted, you're probably not going to get an X, but you may end up in a Volvo S60, Jaguar X-type, or even a 3-series. It never hurts to ask...
My Dealer: X3 every time.
Autohaus of Clayton. St. Louis, MO