Yes you are correct. The base model has a set defined number of vertices. There are 4 base meshes in the library and everything is defined by JSON.
The problem is that one of our developers has only been able to decode about half of what is needed in JSON to work. So far we have the ability to export the base mesh vertices, the polygon and the weights JSON, we still need the morph targets and expressions. This is done by this tool -
We still need to develop the scripts to get the morphs and the expressions. As of now if you were to complete the steps to add this new mesh to MB-Lab, as I did in my testing, you will still get errors related to lack of morph and expression JSON and the character would not be editable - meaning that it would go into finalized state right away.
As for my attempt to try to bring in a new character (did not go well) I think it had to do with bad weight paints, I was using a model that was already weight painted so I only did about 1/3 of what was needed or so I thought. I think I need to start over in the painting but I just had not gotten back to it so I could focus on the current version of MB-Lab. The plan is some future version of MB-Lab will have new mesh in it, just not sure when but it is actively being worked on, little by little.
Once we DO manage to do this, yes that knowledge will be shared because that’s what we do.