I work with a bloke who is your typical country lad, 130kg of angry farmer. If he goes down in a building I can't drag him out, but by christ am I happy to have him next to me in CABA, he is just 110% muscle.
We can't use the BMI, as that would almost count me out of CFS BA, I'm about a 33, why not a changed Beep test, like others have suggested, plus a functional strength/fitness test. People join, let them know the standards (adjusted for age/sex) and then before they're voted in in 6 months, they have to pass it. It gives people enough prep time.
I know that it won't be popular, but at the same time its about peoples own safety.