I haven't played around with the upmix plugins. I thought they worked with phase tricks. ie isolating stereo vs mono content from a stereo pair. And then adding in targeting by frequency band. Multiband MS<->stereo kind of stuff if you will. (You can do this stuff 'manually' too.)
I don't think any of these work by dynamics profile? Or do they? Target different level content in the mix and separate/isolate based on that? This would get into actual DSP tricks vs just passive phase tricks. I thought I saw a "multiband" limiter that did the "multiband" as volume bands across the dynamic range. Never tried it...
Anyway...
If the upmix plugin was targeting dynamics, a volume war style limited master would be ruined for the process. If it's just the usual phase tricks, the volume war damage will still be what it is but the process should still work. There would probably be some extra artifacts but not complete failure.
And bla bla speculate...
Smash something down and turn it up 12 db with an L2 and try it! What happens?