[kovidgoyal]

If you want to do this, I suggest going the whole hog and creating virtual libraries. Create a submenu in in the popup menu of the calibre button (the one used to switch libraries), which can have entries that look like:

Virtual libraries ->
  Create a virtual library
  Remove virtual library
  _____________

  Virtual sub-library 1
  Virtual sub-library 2
...

Allow the user to define a virtual sub library using a simple search (rather than only a saved search). Each virtual library should have two things, a friendly name, and a search expression.

A good place to indicate the use of a virtual library is the title bar. Currently it reads || library name || you can change that when switching virtual libraries to something like || library name :: virtual library name ||