According to this Joomla Magazine Article the improvements of JLayoutFile allow adding custom defined include paths to search for layout files.

The logic behind this is quite self-explaining, but I just does not work, as the render() function calls getPath() to find the appropriate layout file. This method now calls refreshIncludePaths() which resets the includePaths array, so any added include paths get lost.

A possible solution would be to just not reset the includePaths array while refreshing paths, but I don't know if this would destroy some other intended functionality

Tracker: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33618&start=8725