Mutter can't use Gtk widgets in the panel for some technical reason, so it uses a simpler toolkit called
St, based on Clutter. I think you can read a widget's position via its ClutterActor base class, and I think it's even possible to get information about widgets belonging to other extensions. gnome-shell has some useful utility libraries, but some of them may only be available to _javascript_, and you'll probably have to
browse the source code and/or ask questions.
Your code will have to run in an extension to get this info, so if the pseudo-clickers etc are external they'll need some way to communicate with each other.
PS These mailing lists are being phased out in favour of
Discourse.
