officially, no you cannot...the BW parking is supposed to be for hotel guests and those visiting the BW...to visit Epcot, you are supposed to park in the main lot for Epcot parking, or the TTC and take the monorail...
With that being said, we have done it many times before...The guard at the shack will ask you why you're there, and you tell them that you're going to dinner at one of the restaurants on the BW and then going to hang out on the Boardwalk for a few hours...We have never had a problem...