Using a junction, I was able to "redirect" my itunes media folder to an external hard drive without itunes noticing.
With the application "link shell extension" (http://schinagl.priv.at/nt/hardlinkshellext/linkshellextension.html) you can comfortably create such links without using the command line.
For your problem I would suggest either a junction or a symbolic link. I can verify that a junction can trick itunes, but am not sure whether it works well with removable media. Symbolic links even work with remote sources, so if the junction doesn't work in conjunction with the usb drive, maybe the symbolic link does.
With the mentioned application installed, it's as simple as right-clicking the folder containing the media on your usb drive and choosing "Pick Link Source", going to the parent folder of your itunes music library and rightclick->Drop As...->Junction/Symbolic Link, and then renaming the link to (I think it is, but you should verify) "iTunes Media".
It would be a workaround, but I hope it works for you!