Hi,
Currently UIUtilsFuncs contain ui_path_box_new(), so a file-chooser-dialog button can be created programatically in a plugin. But there's no ui_setup_open_button_callback(), so it's impossible to load such a button from a .glade file and setup it, as in Geany.
geanyprj uses ui_path_box_new(), and other plugins (saveactions, spellcheck, debugger, ...) create file-chooser-dialog buttons manually, so they seem common. I'm writing a new plugin, and would prefer to use .glade for the interface as much as possible (and practical).