Question: powering a dummy thermostat from the bus

[jaspervlb]

I have implemented the gateway as in your schematic and used the code. Thank you for the good instructions.

I have however found my thermostat to not implement all features of the OpenTherm protocol. As such I was thinking to build my own. At the location I am intending it is not so easy to provide outside power, nor foresee new cabling. I would like to power directly from the bus. Clearly ESPs are out of the question.

On page 15 of the protocol it is written that 5mA can be taken from the bus. I was thinking how to modify/add to the top left schematic of https://raw.githubusercontent.com/jpraus/arduino-opentherm/master/doc/shield-schematic_rev4.png to harvest power from the bus.
Is it as simple as putting e.g. a 3.3 switching regulator in parallel on the OT terminals? Or will that mess with the remainder of the circuit?