I am definitely a fan of this idea and would like to volunteer my Robin power for the crew battles if you have need of more
However I also wanted to point out that I don't believe the culmination of a couple discussions between two communities (multiple personal experience) plus a single crew battle using players of random skill levels constitutes a strong methodology; between the subjectivity of the first part and the wildly fluctuating variables of the second without a good ten or more trials it's still plenty flawed. Maybe if we could somehow combine the JMU results with results from regionals or majors or something...
But without a better idea myself I'm just gonna stfu and wish you luck 8D
I should probably illustrate in a little more detail exactly what goes down in these events lol.
So when a new JMU begins, a new channel dedicated to the event opens on the character discord server hosting the event, with the other community joining said channel to participate in the discussion. Because the event is segregated from other activity on both servers by being it's own channel, only those interested in participating and learning the match up will join the channel and do so. As such, those who enter are open minded and willing to listen to arguments/counterarguments to the topics at hand, not to mention that biases towards characters are, a vast majority of the time, counterbalanced by the presence of the opposing characters mains and their perspectives, which results in both sides striving to get to the bottom of the inner workings of the MU. I also forgot to mention that both communities can freely have friendlies anytime during the event, so everything talked about during the JMU can be proven/disproven through hands-on gameplay.
You are right about it not being a perfect system, though. Like you said, there are varying levels of skill involved in JMUs, and the best players aren't always available to lend themselves towards the crew battle at the end, as some communities will often prioritize including players who haven't participated in a crew battle over simply reusing their best players from the last one. However, something that needs to be remembered is that the crew battle isn't held as a "winner decides the match up" sort of thing. The crew battle is primarily held as a means to showcase the execution of everything learned from the week prior (JMUs are generally week long events) and to provide gameplay of several different players to review together (after all, different players will play the same character in different ways, and thus will try different things). So yes, the variety of skill levels can cause wildly different results than would be expected from rehashing the servers best players each time, but I believe it's made up for in playstyle variety, which I feel weighs more in determining match ups overall than mere player skill. Playing a matchup against a better player will only really tell you that the player is better than you, not necessarily that the character is. It's vitally important that we can make a distinction between player and character merit.
At the end of the day though, whether the method is flawed or not, I'm certain that this MU chart will be a helluva lot better than you're average personal MU chart we all see players posting so much, lol.