We didn't use Whitehall but for our reception we paid the valet at the Beach club for our guests. Our planner made cute park tickets with our wedding info on the back to give to the valet and then we were charged for the amount of tickets they received at the valet. It included the valet fee and tip. We were charged an estimated amount for them on our BEO and it really made parking easy for our guests. It's the one thing at our wedding that I don't have a pic of and I wish I did.
Maybe you could do that for your guests? They could park at the GF valet and then walk over to the convention center which isn't as far as parking in the self park lot then walking over.