I'm afraid that as of today, there is no way to send non-JSON payloads
out of ThingStudio on an MQTT publish feed.
You can do this in HTTP feeds, because of the Feed Processor feature,
where you can change the incoming and outgoing data to match legacy
or fixed API's, we just haven't back-ported the Feed Processor mechanism
to MQTT yet, although we intend to do so soon.
So sorry, not just yet, but we intend to allow exactly this for precisely your reasons.