Several years ago we were at WDW over July 4th. The Magic Kingdom does the July 4th fireworks on both July 3 & July 4th. We watched from the Polynesian beach on July 3 so that we could be in Epcot on July 4th for their special fireworks. I would just leave your car parked at BBQ and use Disney transportation to reach the Poly. Just know that once the fireworks are over you will have a long trip to get back to your car. No trouble getting out of Poly but you will have to go back to MK to catch boat back to BBQ and will run into all the folks leaving the MK. You might be smarter to get on the Resort monorail and take it to the Contemporary. There you can catch a boat to the BBQ location. That way you will not have to change transportation at the MK - but you will be squashed from the MK to CR with all the folks that get on at the MK.
Another exit would be to walk to TTC from Poly after the fireworks and catch a bus to Fort Wilderness. [I think that is where the BBQ takes place] Maybe the bus could let you off right where you left your car at the entrance to Fort Wilderness. This plan eliminates the need to use the internal bus route at Fort Wilderness to get back to your car. This only works if Fort Wilderness folks still have to go to TTC to catch a bus to Animal Kingdom, or Disney Studio. Now days they may have direct bus service.
