I think they are just mesh swaps..
So you're changing the value number of one in game model for another's so that it plays using something like Trigon instead of Raven then? Probably can only access models attached to the characters it loads into the stage then thus the Amazons for WW and Billy for Shazam.
I'm guessing since its stored on the disc you cant simply edit the game file to host any new non-NRS models that aren't on the disc already then unless you can figure out where its accessing DLC character files from and edit their data on your harddrive then.
Am I in the ballpark here? Sucks if you cant make your own models for this and load them to your console to take advantage of whatever you are doing to get custom cast alts.