Since this took me a while to figure out the best method, I thought I would share.
If you are posting JSON data to MQTT and want to use a property of a topic to populate a label:
Posting JSON to MQTT:
Getting a value from a specific property of a topic:
Annoyingly, without the ability to set the retain flag and QoS in MQTT, UIflow's support of it remains just a toy.
In addition, we really need a flag that tells us when the MQTT connection has actually started because there seems to be a significant delay before it works.