It would look/feel nice and easy to use if geanyprj plugin would list project file as a treebrowser from base path. Instead of what it does now i.e. list all files with base path in a scrolled view as a text label, it would be nice if it implemented the sidebar part of gproject plugin.
Have you already had a look at the treebrowser plugin itself?
http://plugins.geany.org/treebrowser.html
Maybe this is an option for you. :)