Not *great*, but a start.
MessagingMenu has a function for mapping an app to desktop file data. This relies on GLib to parse the desktop file,
which needs the binary from the 'Exec=' entry on PATH, otherwise it bails.
messaging_menu_app_set_desktop_id calls g_desktop_app_info_new:
334668ad5a/libmessaging-menu/messaging-menu-app.c (L245-L252)
Which, after some jumps inbetween, ends up in g_desktop_app_info_load_from_keyfile and calls g_find_program_for_path:
204a76669b/gio/gdesktopappinfo.c (L1920-L1951)
Which uses PATH:
204a76669b/glib/gutils.c (L371)
93e91c150e