At the HP1000 driver level, you could get the Python source and add in the MQTT code yourself. The Python MQTT libraries are fairly straight forward (in my experience) but I suspect there will be issues with coordinating the timing of the two data sources. You could have a thread that subscribes to the MQTT topics you are interested in and buffer the last received value that can be picked up by the HP1000 driver and added to the packet passed on to the WeeWx main thread.
Of course you will also need to make sure that the WeeWx main thread and database know about the additional variables but you would face that issue anyway.
Susan (author of the HP1000 driver)