Auto-Import is wonderful, and all that, but seriously, I don't let it watch my main library files.
I have a lobby here, a waiting room, if you prefer. It's just an "incoming audio" directory. MC is set to rip there, downloads are are directed there too. Otto watches that and imports anything that lands there. I have a viewscheme set up that only shows files in the waiting room. Once these have been checked over and tagged correctly, I use the rename from properties tool to move them into the "library proper" beside all the other audio files, and that is the last Otto sees of them.
I genuinely feel it's better that way. I can do file upgrades and such like within the library proper without Otto intefering mid-way through the process.
That would take care of Otto for you, I'm not sure about the rest. When you say disconnecting is a PITA, I take it they're not internal hard drives? so, External USB ones? or mapped network drives?
I can recall just one time I wanted to change the field without moving the files, and back then, I just made a new directory and dropped the existing base directory into it using explorer, thus breaking every link in MC's library. After making the adjustments I needed, I just moved them all back again, putting everything back where MC expected to find it. You have to be more careful now though, because if Otto is watching your entire library, and he gets wind of what you're doing and can't find where you moved the files to, he'll empty your library for you!!!
I do remember, a looong time ago now, reading a post on this subject. Back then, the reason was for taking the library to different machines with different drive letters, or something along those lines. Was that you? If so, would you mind refreshing my memory?
-marko.