Using clues from other users from all over the Internet, I have been able to find out the MQTT message topic/content to turn on/off the remote climate control on the Nissan Leaf. I'm using on Node-RED, but this is applicable to using MQTT in general.
Message topic: VEHICLEclient/USER/command/homelink
To turn on the remote climate control, the payload is: homelink 1
To turn off the remote climate control, the payload is: homelink 2
The VEHICLE string is the ID of your vehicle. If you don't know how the OVMS is sending it to the MQTT server, subscribe to a topic from the Leaf, and it will be VEHICLEmetric/xnl/cc/remotecool - use the same VEHICLE string on the publishing module.
The USER string is the ID you use for the V2 server configuration.
I hope this saves some time to anyone else using OVMS and MQTT!