I'd suggest the OP contact the organizers of any ride he's interested in and ask the implied question, "When do your rest stops close?" Most every big ride I've been on has had sweep SAG (at least one of them annoying; I was last on the course when the group I was with quit at 55 miles!). They'll normally be in contact with the rest stops ahead and let them know how many more riders are on the road.

And, as long a the OP can keep pedaling, it's fine for him to do so.

On the flip side, the organizers and volunteers have a reasonable expectation that the riders will maintain some average pace. 10 mph is on the low end for any ride I've been on. Good rides normally post their closing times at the finish. It seems reasonable that anybody who's out on the course after closing should expect to finish at a deserted parking lot.

OP, how long have you volunteered to stay at a rest stop or finish line? You know these rides only work because of volunteers, right?