good question
I think the weigh-in in wrestling is actually part of the problem not the solution...if I understand it correctly you stave yourself and dyhrate yourself - weigh-in and then go eat and drink and wrestle later in the day at a higher weight. Isn't this sorta cheating the weigh-in system.
to do it properly the weigh-in should be done ring side right before the match -with no chance to go eat and rehydrate...this makes you wrestle at your actual weight. you want to wrestle below your true weight - good luck doing it in that condition.
this solution for CC would create more problems than it would solve -
1) in wrestling its main purpose is to have competitors of the same size/weight competing vs each other because it would be highly in favor of bigger/heavier wrestlers vs lighters one....there is no such thing in CC/track - about height or weight.
2) if you are underweight as a runner - that is your problem - you live & race and train that way - if it helps you in the short time - that is your choice - it probably won't help you in the long term....I'm not sure I believe it really helps you in the short term - but it could.
3) a better solution would involve education than punishment.
4) the real competitive disparity in our sport has to do with drugs! that is what we need to focus on first.
finally if you are going to pass some BMI requirement is has to be for both men and women and what are you going to do about bulimia? mouth swab before competitions? this is going to get pretty ridiculous