I agree with you that it would be ideal to insulate behind the ply but with the solid partition hardly any heat from the front cab area even makes it way into the cargo area.
I've placed a remote sending thermometer in the cargo area with the receiver temperature unit in the cab so I can monitor the max/min temperatures experienced. Unfortunately, as a for instance if the outside temps are -15C and I am driving with the heat on full blast, after 40 minutes of driving the cargo area would only "warm" up to maybe -10C.
Yes, with insulation added it would probably warm up in the back a little bit more, but with very few to no venting spaces in the factory partition not much warm air would make it into the back area anyways.
Here in the Prairies, when it is winter time the air is so dry and rusting of tools and such is really a non issue. It's so dry that your hands and lips crack and you get nose bleeds.
Attached to the forced air furnace in our home is an inline humidifier so that at least the home humidity is at a normal level. I've seen homes that have a malfunctioning humidifier have huge gapping between hardwood flooring planks due to shrinkage from low humidity. Plus, without humidification whenever you touch a light switch or any metal surface you get a huge static electricity zap constantly!
Even if insulated, with the cargo area being so large it would cost a small fortune to heat that space with an electrical heater so that's why I'll limit the heating to the hot box chamber mentioned earlier in the thread for my paint and caulk consumables.