So the dog wears a harness, and we have a restraining strap that clips onto the harness and then buckles into the seatbelt.
Makes sense. I would go to a junkyard and get an extra female seatbelt for your make of vehicle, and just bolt it in in tandem with the front passenger belt. You shouldn't even need a new bolt. I'm pretty certain that rear seats, which have the center belts sharing the same bolts as the inner sides of the outboard belts, don't take a longer bolt. There's enough crush space in the carpeting to make it possible.
Technically, I believe it's illegal to install used seatbelts, and some yards may not sell them. Since you only want it for a dog, and you wouldn't be installing the matching male end for a human passenger, they might make an exception.