I'm not seeing any other way either.
Would definitely make you dependent on adjusting your monitor volume at the monitor in split mode -- otherwise you're going to be changing the level sent to FOH every time you change your monitor volume and its a preset parameter, not global, so if you're using multiple presets....ugh. I guess you could apply the same modifier to preset balance in all your presets and use an expression pedal, but who wants to dedicate one of the two pedal inputs to that, and also still doesn't fix the "one goes up, other goes down" issue.