Yeah, I hear you. Obviously not trying get into any legal troubles.
I guess in my head the mod would simply cycle through default costumes, purchased/unlocked costumes and modded costumes, and a random color. Perhaps like you said, it only searches through default and patch directories and leaves the dlc directories alone. And anyone who wants to include their dlc costumes into the random costume selection process can copy them to said patch directory?
I know I said “simply”, ha, but I imagine this will require some sort of script.
Maybe an alternate solution would be just to create a totally separate folder for the script to read from, with the same character file structure, but all the files are empty. And if anyone want to add costumes to it for the cpu opponent to select from during arcade mode, they can add the costumes into that folder manually. If all the folders are empty, then the cpu selects the default costume with a random color. Seems like the easiest solution.
Btw, I’m not sure what the file structure is for the Upgraded Version of Ultra in regards to dlc, I purchased the Full Retail version of Ultra with all of the costumes unlocked.