Rim is supported in
SpatialMaterial, all you have to do is to enable it. It works in editor of from a script.
It shouldn't be too hard to do it with a bit of scripting:
Get all materials of your character, and enable rim on them:
for i in mesh.get_surface_count():
var mat = mesh.surface_get_material(i)
mat.rim_enabled = enabled
# + set other params if needed
However that will modify all
MeshInstances if you re-used the same mesh multiple times.
Another way is to use
material_override, which will temporarily use a different material for only one particular
MeshInstance. So you could duplicate the mesh's material, set rim params and set it as an override.
However, if your mesh has multiple materials it's a known problem: https://github.com/godotengine/godot/issues/14588
Apart from that, I'm not sure what you want to be added in the engine, the effect you posted in your screenshot can be achieved already. If what I said doesn't help you, please explain your problem better because I don't understand half of what you wrote...
If you can't find a solution anyways, you can go to the Github, do a search, explain clearly your problem, why you can't do it and why it should be built-in.