Nice idea, and folks wont be able to farm wins very effectively if you use something like MMR because those kinds of ratings only change substantially in response to unexpected outcomes. If you were to set up a burn account, repeatedly smash it in a custom lobby your MMR would go up a bit, but you would suffer serious diminishing returns because the burn accounts MMR would drop to the point where new matches against it (which weren't losses) wouldn't change your rating much.
The problem is that long term the visible MMR of this new system would end up matching your actual MMR, which Muse have tried to keep secret. That plus the underdog system is enough information for me to reverse engineer how the matchmaker works which Muse have said they don't want happening (technically I could do it now with the underdog system by keeping a hand tally of results in game, but in practical terms this is a lot of effort).
One way to avoid this would be to have the tournament be short, since then the tournament MMR is less informative, but then the tournament MMR is less informative.