Looks like @Dan_Petit is running a zone using Windows, and I haven’t tried the extension on this. There’s no technical reason it couldn’t run in a command window, and as long as the key presses come through in some sensible way, it should in theory work for basic transport control. But clunky, and would obscure the Roon UI (I assume Dan has this on a TV/Monitor via HDMI?) as it would have to be foregrounded to work. It would be much better if Roonlabs could add MCE remote transport control to the main Roon UI for what I think he wants to do.
Of course, if Dan wants to buy a dedicated, headless Pi (a £10 Zero-W should do the trick) to pair with the Harmony via Bluetooth and control the zone, that would work - the extension doesn’t actually have to be installed on the the player.
However, as I mention in the instructions, it does require a willingness to work on the command line in Linux to get it all set up. Installing the extension is easy, but pairing the Harmony remote is fiddly. The Harmony programming software isn’t great, and neither are the Pi command line Bluetooth tools. It is a once off thing though - once set up, it just works.
Currently the extension doesn’t do Roon volume control, but that should be easy to add. I’m thinking of doing it anyway, as I’ve just bought a Pi/iQaudio DigiAmp+ for the bedroom and will need it for that, as there’s no separate amp with IR volume control.
It doesn’t provide any of the searching/browsing/playlist building Dan wants, but then again I’ve never seen a UI that does that well using just direction arrows on a remote. Plex gets closest but is still a pig for large music collections. I love a remote for basic transport/volume control, but personally would always pick up a tablet/phone/PC for anything more complicated.