I didn't want to go into too much detail in my first post about my failed plan-finding experiences, but now that you brought it up...

Neuschwanstein: I think I have a copy of just about every high-res map and photo out there. The plans shown on the castle's official website are but tiny versions of the large scans I already got from a book; yet they're incomplete. When I e-mailed the official site to ask about plans, I was basically told that they're a state secret!

Disney: I've never been through the inner workings of the Disney castles, but I've done my fair share of Internet research on them. Yes, I believe there's a restaurant and a small "princess" suite in Cinderella's Castle, but I don't think there's anything anacronistically-functional behind the scenes. Simply put, what you described is pretty much all they can squeeze into the place.