Oooh, i was once writing a general purpose editor. What i did was, read extern skin names from MDL7 files, write those to a metadata file(json) along with model name and content/model browser would use those files(also had thumbnail/preview images and other data about files) to show the list of available models. I think you can use a similar approach, either create those files if you have small amount of models or create them with a little script automatically. And use one of the entity skills/strings to remember which metadata file that entity came from and use that to get skin names.

I also remember converting internal skins to extern. But i remember some of those was not available in mdl7 sdk so i manually did some stuff, which ones were missing i do not remember.