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.
Issue comments
Also, sorry for putting it under bug report. I thought it might be a bug also since the armor model seems to not obey the movement you assign it to ("no animation") when you assign the whole model to the leg or something of the sort. ("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.")
This does not make this a bug report
Only one body part can be specified so this is not possible unfortunately.