Good question.
There is a fishmonger round here who delivers on a Friday and drives slowly with his back doors open (can't remember if it's a sprinter or a transit) because he is in and out of the van so much, I'm not sure how he secures the doors because he doesn't usually have them open fully 90 degrees.
The basic checkstrap will hold the doors at 90 degrees open, whether it is strong enough to use on the move or not is another question, 180 degrees is not much use, you van would be twice as wide as normal. 270 degree opening onto the magnets (mine are missing) would hold the doors open for minor increase in width, again not sure of the security on the move.
The final, and perhaps most important consideration though is about your structural rigidity, there is not a lot of transverse structure in any van to stop the walls and roof racking and I have a feeling they are designed such that the closed doors provide the lateral bracing necessary to prevent this mode of collapse. You might be safe enough if you stretch some ratchet or cam straps diagonally across the doorway from bottom one side to top of the other and secure them tightly in an X so each can work as a tension brace (be careful with ratchets not to over tighten and distort the frame, those things can be powerful!).
A friend of mine built a trailer to carry kite buggies using unistrut on an old folding camper (posh trailer tent) chassis. In order to get 4 buggies on he made uprights that could slot together as the layers go up, but of course the buggies themselves prevented fitting any cross bracing. I commented on it but he seemed to think it would be OK - the buggies have 2" tubular rear axles and were secured with jockey wheel clamps so it looked pretty rigid. We followed his son a short way watching the load sway dangerously at corners before flagging him down. Fortunately I had put a bunch of cam straps and ropes in my bag because I wasn't sure how the buggies were supposed to attach to the trailer (they were still finishing building it when we turned up, we had a 9 or 10 hour drive down to south Wales for the euros and were going in convoy, with our buggies on the new trailer). After 10 minutes of weaving diagonal ties this way that we set off again with no sway - we did re-tension some of the ropes when we stopped for a meal but when we arrived at the venue in the early hours of the next day everything was still solid. Unfortunately I can't tell you exactly how strong the straps would need to be, or how tight to effectively brace the back of your van, I can only anecdotally tell you it can work well.