Simply add the entire mosquitto message as a JSON entity, not a string.
The simple case now looks like:
"mqtt": [ {"topic": "/s/cmnd/5C:CF:7F:20:29:6E/Power", "message": "On"} ]
Sending an RPC to Mongoose OS looks like:
"mqtt": [ {"topic": "esp32_XXXXXX/rpc/Button.Touch", "message": {"args": {"idx":0}}} ]