That is why I asked what spec the cage is built to. We don't tell you the exact spec for the cage as it has been done by so many other organisations why re-invent the wheel. For winch challenges my preference is ORANZ rules as they are the most relevant. ie offroad, a bit of speed, comparable vehicle types. ORANZ cages are designed to deform in a high speed crash and save your life, not save vehicle damage. This is the same for MANZ and CCDA (Oz) spec's. This is where I personally think the NZFWDA rollcage rules are not quite right for us. Those cages are more about rolling over and being able to keep running. Sure the cage is strong but it is actually too strong so in a higher speed crash there will be no deformation to absorb the 'G' forces so it will all be transferred to the crew.
Note: the NZFWDA cage coment is a personal opinion, not an official 4x4 Challenges position.
So if you look at any of the above rules, any cage whether 4 or 6 point requires diagonal bracing. Just because it has been accepted in the past doesn't make it right! Through no real fault some things that have happened in the past weren't 100% correct or safe and we want to make this as safe as possible.
So your idea about changing back stays and including a diagonal is spot on, but I would also look at changing the bar above the door. I would shift that so that it runs at the top of the bend and then your back stay will be higher up the main hoop. This will also support the ali sheet you are planning to put on the roof. The rules all state that the back stays should be as close as practical to the top of the main hoop. (does all that make sense?)
When it comes time to get rollcage compliance for a log book, you will need to tell us what spec the cage is built to, what it is built out of, who designed it and who built it. We'll be at Nor-West to sort this out if ou want. It will cost $26 to join 4x4 Challenges and get your logbook. It will then cost $6 per year membership ie $20 joining fee that for the next few months will include your logbook.
Ultimately it is your life and we are just trying to help you guys help yourselves.
As for your door idea, that is what I would do and as far as I understand the rules it perfectly acceptable.
Finally your guys attitude towards this is shit hot and is exactly what this sport is about and what we are trying to keep while growing the sport!
The best thing about WC is the attitude of the competitors and you guys seem to have it spot on. So we want to watch you guys give it absolute shit on Saturday night as you are our entertainment while we have a couple of cold refreshing beverages!!!!!! 
I promise to provide some entertainment of Friday night for you as well!!
Re the cargo barrier (or lack of) we will not be turning anyone away that doesnt have one provided all the gear in the back remains totaly secure ,I would suggest a toolbox bolted to the floor for winch block spare shackles etc ,These MAY be checked during the comp as a few years ago there was a case of a competitor competeing without a cargo barrier who after doing a 2 to 1 winch did not secure the block in the rear as it should of been they did a big nose dive and it flew between the driver co/driver on its way through the windscreen ,This is but one of the reasons this rule is in place..Steve
even won one once... 
