Is it possible to automate adding a layer to hundreds of existing models in an assembly?
If your top-level assembly has a layer with rules, you can "extend" it to all sub-components that do not have such a layer, meaning it will be created in those sub-components along with the rules. This function is in the layer tree, in the dropdown decorated with the "stack of layers" icon (next to the hammer + wrench icon)
I forgot an important detail, the layer added will be different for every model (it would be the model name).
That sounds very non-logical...
What are you trying to accomplish with those layers?
CAM software uses layer names instead of model names. So when people import an assembly they can see what the name of everything like we see it as model names. A solid geom layer with model name as layer name would import into CAM.