That's weird that the RME UC doesn't allow system volume to control it. My RME Fireface 400 does. I use my wireless keyboard volume controls all the time. Whether or not I'm using a DAW. It controlls the fireface output level which in turn controls Ableton's output (main outs going to fireface) as well as system volume. However there was a brief period when I upgraded a bunch of stuff like the OS where it didn't work. Before I could figure out why, it started working again as expected. I don't know why, it may have been after a updated the RME driver, not sure. In any case, if you're using RME, make sure to download the latest drivers from the site.
Professional Shark Jumper.