I think MANY players skip out on corps just because they fear getting a fuzzy tone.
I just say bring along a concert horn if possible and combat the fuzz.
An F/Bb mello could be useful, but a heavy monster.
the fundamental pitch of the F mellophone is lower than a Bb instrument, thus sorting out the partials up high is harder on the F instrument. I could kiss the guy who brought in the Bb side for french horn.
WGI has listed rules for em (I'm told), so why aren't there any?
Personally, I think it's a great idea.. Many organizations let their horns collect dust over the winter. :(