You've got a few options here. FWIW - I've rarely been turned away from Boardwalk. The times I have were due to a conference taking place and peak times (such as the 4th). If you have a res - you should not have a problem.
For future reference, you can also "visit" Yacht Club/Beach Club or the Swolphin.
Swolphin will charge you to park but there may be a way around it. Go onto the Marriott website and make a booking at one or the other for a time frame out in the future. Make sure its not a prepay and it can be cancelled. Stop by the front desk and show them your res, and tell them you wanted to check out the place before you stayed there. 7 times out of 10 they'll validate your parking ticket. But you have to ask.
In a real pinch, park at DS and take a bus.