Have to ask a question.
Is there a practical reason you want to do your player models this way?
There are a couple of reasons you would want to go this way, one being the need for procedural NPC’s, but taking into consideration that all player models can use the same rig and blueprints it’s rather redundant.
As for direction the ideal would be not to think in terms of making clothing fit a given player model but rather what can be added as a target that will allow clothing to be composite onto as many different player models as possible.
First bullet point is morphs are not an absolute but rather can take shape based on a percentage value and other shapes can be applied as an additive value to the current shape.
Taking your example we will assume that your muscular model is bigger and has longer arms so to make a coat fit and is usable for other models you need two targets.
- a target with longer sleeves
- a target with more volume
Since you can set a morph as a percentage the jacket will fit most if not any other player model that has longer arms or more volume as referenced by you base model.