I don't know for sure but I suppose you can change the defaul library icon using resource hacker on explorer.exe or imageres.dll, probably one of the two. That would require taking ownership of explorer.exe at the least, and there's the possibility that windows senses the change in the file and silently changes it back to default.
Also I suggest using converticon
to generate a "standard" icon with all dimensions, otherwise your new icon may not display correctly on all sizes.
If you just want to change single library icons, this program
will save you the need to manually open library files.