Rear A/C is a very common Club Wagon option. You should be able to find enough donor parts to cobble something together. They normally live where your shower unit is but you should be able to move it around without to much grief.
Have you checked the build on Hal the van? It probably has a front, mid and rear A/C-Heat system in it.
Gnarvan 1992 E350 Clubwagon
Advanced 4WD Systems Conversion
Sotar Legend 12.5' X 22" Cataraft
2003 BMW R1150GS-Adventure
2013 BMW R1200GS-Adventure
Assumed I would have to pull a system from the junk yards.
Having it in the Club Wagon I just sold made me realize how important it will be for the current build.
The front only barely kept up going through the desert last week when it was 110 out. Still don't have the interior finished with insulation etc but doubt that would make that much difference with that type of heat and air volume in the van.
I have a brand new, never installed Ford Club Wagon system complete with heater core and evaporator---just as you'd order from the factory. PM me and I'll give you full details---price would be about $200.
I had my factory rear air removed from my 2009 E250 cargo van. The local shop that did the removal probably still has all the parts available. It's not just the rear unit - there are some different fittings on the refrigerant lines under the hood, plus the piping going to the rear.