I'd have to guess that this could just be a " free for all " ...no smoke / fire damper issues , no outside air requirements...just " run what you brung ". As long as the structure penetrations are firestopped I can't think of anything.
I would definitely look at some sort of minisplit system, rather than place ductwork on the exterior of the home. Seems like you could also do a PKG unit and install ductwork in the corners of the walls at the ceiling and drywall frame around them. But that might be worse yet.