I created a Global Action that shows a VF page, and when I used the Global Publisher to add that action to the list, it works in most places (Chatter, almost all object types). The one place it doesn't seem to work is on the User record. I can edit the Page Layout to remove standard Actions, but custom Actions seem to get ignored regardless of whether you inherit from the Global Publisher or customize the User Page Layout's Action section. Has anyone else seen this or figured out a way around it?
I have not personally tried this, but I know the User object isn't "normal". For instance, it doesn't support workflows, or inline editing on the page layout, and has locked areas on the page that can't be moved around. My suspicion is that this truly is not implemented yet, but have nothing to confirm that.