I honestly don't think they care if you stay really late, they just don't want you "sleeping" in the overflow lot.
If you wanted to close down Jellyrolls and catch one of the last buses back to Ft. Wilderness then you can do so. You are a resort guest until the last venue closes for that business day (meaning somewhere around 2am the next morning).
We use to do this all the time. Your parking pass is also valid at the parks on the day of check out. So we have been driving to the park. I don't know if you are in a Motorhome or are pulling a TT but that is not a problem. We pull a 35' TT. It is handy to have snacks and cold drinks there. At AK campers are directed to park up nearer the park entrance with the charter buses. Other parks will usually direct you to park at the end of the next unused row in the parking lot. Sometimes, at the end of a long day it is nice not to have to take transportation back to FW before hitting the road.