You can wake the car up and start the climate control by sending a frame or two on the EV CAN bus, see https://github.com/openvehicles/Open-Vehicle-Monitoring-System/blob/470af78c4b9315da0062d2fa2bc5d3fdd7561822/vehicle/OVMS.X/vehicle_nissanleaf.c#L405
which implements remote climate control in the OVMS project.
Gen 1 cars require a wire to be toggled to 12v to wake up the system before you can send the climate control command. I unplugged the TCU on my Gen 1 Leaf and connected the OVMS to it's plug to access the EV system activation request signal. See https://carrott.org/blog/archives/158-OVMS-Nissan-Leaf-Remote-Climate-Control.html
I've had a report that a Gen 1 car doesn't need the EV system activation request signal, so maybe there are some Gen 1 firmware versions that implement the pure CAN bus wakeup that is present in the Gen 2. I haven't tested many Gen 2 cars, but I have found that not all Gen 2 cars work -- I suspect cars with a TCU installed don't work because the OVMS sends "turn on" and the Nissan TCU sends "idle" but I haven't had a chance to test this.