When I was working at Neighborhood Bikeworks in Philly (youth bike program), one of the kids wanted a tag-along for riding with his dog. Rather than ordering him one, we attached a quill stem to his seatpost, and attached an old fork to that. In the fork, we put a QR hub.

The kid trained put the leash around the hub and put it in the fork, and trained his dog to run with it. Dunno if he still uses it, but it worked for a while.