I think that without a long and drawn out fitness test, the BMI is probably the best way, at the moment, to do things.
For people who fall between say, 30-40 BMI perhaps need to take part in a VO2Max test? but then this would have the issue that people with a 'normal' BMI could well score very low on a VO2Max test.
Perhaps the answer is instructors having more balls to mark people NYC based on their fitness performance during the OBAOC course. The conditions replicated in the Hot Cell are certainly mild interms of both temperature and intensity when compared to real life operations. If people can't hack it in there, then they shouldn't be deemed competent to pass the course.
As much as people might whinge and moan about it 'not being fair' I don't give two hoots about being fair, as long as I can rely on the bloke next to me and I know that they are capable of performing the tasks assigned as well as any worst case scenario. Its much like people being accredited with RCR when they can't handle the tools physically by themselves. Its a terrible practice of "Oh it'll be fine".