This would be the best way to gather actual feedback on balance changes. Testing stuff on paper, by individuals of different experience levels, only provides a very limited perspective of how each change affects gameplay and leads to varying personal conclusions (and more often than not, arguing).
I don't know however if an organized event, targeted for competitively involved players, would be doable. After all, there hardly ever are enough people answering muse's calls for broader, more important dev-app tests. Anyway, I think it's a good idea and it's worth a shot. Better to test stuff early and discuss changes with each other rather than wait for them to be released and yell at muse afterwards.