Well after those initial tests I now finally have my Hydreon rain sensor full set up and integrated.
I've ended up connecting it to the motor through my existing relay board, via a NC relay. The relay board is on UPS just as the Hydreon is. It means that I can disable the Hydreon if required by flicking the relay to Open but by default it will function without PC connectivity. I've done this because I'm not 100% sure how the Hydreon is going to behave a after rain and with heavy dew, so at least I have complete control remotely still.
https://www.dropbox.com/s/r7pzt66puq...%20pm.mov?dl=0