Hi Phil,
The master zone should output 24v like all the rest of the zones. If you are getting 1.5V there may be a problem with the board or somewhere in the circuit.
The software can switch things on and off but is not able to control the output voltage. That would require additional hardware support.
There is a plugin system for adding software features. You can look at the files in the OSPi/plugins directory to see how they work.
Dan