I can't touch seats or seatbelts, my insurance carrier will not allow it at the shop. If you modify your seatbelts, ... your insurance won't like it and you are likely open to a lot of liability if you ever have an accident and someone is hurt, ... and your insurance company will probably involve you personally via contributory neglegence.
That having been said, the only way I can see it being done would be if there were a beam between the rear shock-tower mounts where a normal-length shoulder harness could mount at seatback level in the center. Mounting a long shoulder-harness at the floor level puts the belt at a significant disadvatage in that it will stretch significantly, and it will pull the passenger downward in a forward collision.
The center lap-only belt is still better IMO than the D1 jump-seats, ...