I spent some time with a friend's Gen 2 Nissan Leaf and got to the bottom of why the remote climate control works on some cars and not others.
It turns out that if you have CARWINGS (now called NissanConnect) hardware in your car, the Nissan TCU module sends "Status" (0x4e) and "Idle" (0x86) messages on CAN frame 0x56e. These override the "CC On" (0x4e) message (0x4e) I'm sending with the OVMS and shut the car down.
If you have a TCU module, the solution is to unplug it. I haven't noticed any problems doing this, I unplugged my TCU (on my Gen 1 I had to do this to access the hardware wakeup wiring) about 6 months ago. My friend has been going a few days in his Gen 2 with no problems.
Unplugging isn't terribly difficult on a right hand drive car, you have to remove a panel above the driver's feet and then contort yourself in to press the release latch and pull out the big plug. I didn't bother unplugging the antenna and USB connection to the AV unit, and I coudln't work out how to release the entire module without taking more of the car apart.
I do notice a check engine light when I use the remote climate control but it goes away after your turn the car off an on again and doesn't affect the performance of the car. I'm guessing something notices that the OVMS isn't faithfully emulating all the TCU functions. If anyone has a working NissanConnect and could capture the can bus for an entire remote climate control session, from startup to shutdown, maybe I can get rid of this problem.