I put this under bug report since this should already be somewhat of an option. The problem I had was that custom model leggings only allow for a left and right leg model, but not for a body model since most leggings overlap over the body. I made overalls and noticed this because I was depending on using the body for the model as well. For now, I will just set it as the full model, but in the future, we should have also a body portion of the model allowed.
EDIT: Also, it would be best that the custom model body is not hidden when a chestplate is worn or there is an option on how you want the armor to behave in that fashion.