Here is a “dirty trick” which works in many clients without giving a visual distraction:
Edit the library names and prepend a space character to the library name.
You can use several space characters.
The library with the most space characters “wins” and is placed first.
[edit: if you use 'zero-width space’s instead, it looks good even on clients which don’t support this trick
http://www.somebits.com/weblog/tech/zero-width-space.html ]
