Might sound like a daft question , but do they also make a version where the hinge is at the front edge ?.
Yes. The model years 1968-1971 have the poptop hinge fitted to the front of the van with the luggage carrier in the back.
However, IIRC, if you had your own poptop taken off of a VW camper, you can install it any way you want, front or rear facing.
Also, In the '68-'71 VW, there is an approx 3.5 x 3.5 ft square opening that you get up to the pop top with. The entire roof of the van is NOT missing. Therefore, if you had a 2 x 2 opening cut in the top of your Sprinter (like a vent/fan combo) you could have yourself an access area. You wouldn't *need* to access the pop top from the outside/back.
Good luck!