LOL a jacked mannequin that's funny but to be honest I actually think that could work but I always thought you needed to have like a clay mold or something. Ask Seahunter his skin is beast he should know.
No offense, but that would be disastrous and a waste of latex. The main issue is that peeling the latex off would be a bitch to put it simply, and likely result in destroying whatever layers of latex you have down.
At the very least, you'd need to put a lycra suit over the mannequin first, then latex over that. You also need to make sure that the mannequin is exactly your dimensions everywhere or larger than you everywhere, as latex shrinks right down to whatever you're applying it to. Unless you're a small adult or a younger person with a smaller frame, it's probably going to be too small, as most mannequins are not made to the dims of an average person. If it's smaller than you anywhere, i.e., knee joins, elbow joints, waistline, etc..., you'll never get into the suit and you'll have wasted your latex.
I have a large mannequinn you could try it on my costume dosent fit it 100% so its much bigger than the average bear. buttttttt you would have to buy it cause its my last one pm if intrested. you wouldnt have to do the whole ductape thing.