When I mix and master I always save the mix project, render it, master the rendered file and save that mastered project. So maybe it would be a useful exercise to start with the rendered stereo mixes pre-mastering and treat them how you suggested. The problem I think I will have is I always choose mix presets and then mastering presets to precisely match the song. I'm not really doing proper mastering that way, instead, just using Ozone as another final enhancement to the song's mix. These are very powerful tools, especially in the hands of someone as unqualified as me!