Well, for anyone interested, this little program can work very well in overcoming iTunes' multi library deals.
The way to go is to set it to switch the prefs and the two library files, do it and set that up, then switch back and make whatever changes you need to the original and so on. Those 3 files get switched very nicely.
I used it having put my compilations folder back onto a laptop. I left the vast bulk on an external 2.5" HD and pointed iTunes at the new compilations location. Then built a new library with prefs just for the compilations for when traveling without the HD.
It's fiddly but it's doable, and this app makes it all work just fine now. Back up and go for it.