Can't you directly subscribe individual tags to MQTT payload?.
If Not:
https://docs.inductiveautomation.com/display/DOC79/Datasets
Check the above link. You have to extract each value to another history enabled tag. But you will get new timestamps (timestamp for remote timestamp too). Finally you have to trigger and run the script too whenever the payload arrives. Looking for a payload to arrive, strangely defeats the very purpose of MQTT publish/subscription protocol. For me, it looks like some sort of crude data butchering with a blunt knife 
This is an unviable solution for production deployment.
The best solution is, build your own Java MQTT driver module for Ignition.
If you want to save the JSON payload into a text file and import it directly into the historian database along with remote timestamps, check this topic. Later, you can consume that data through query tags. I think, this can very well be done outside Ignition through some PHP scripting. Just "the other way around" solution.