There's still time before the release of Poser 5.0 , so try the Poser 5 wish list and your idea will be considered, if it hasn't already been suggested.

Personally I would recommend against further nesting of folders, but would suggest instead an expanded libraries option, where Poser can internally select alternate folders in the libraries folder, i.e.

• character2
• pose3
• prop4

instead of just single character, prop, pose or face folders, e.g.