Easy solution is to get a blackmagic ultra studio mini monitor for about a hundred bucks. Otherwise you could use the HDMI out and treat it like a third monitor and put your Project Viewer on that monitor fullscreen inside of FCPX.
If you're using legacy FCP I'm not sure how that would work. I think the Mini-monitor still works with FCP7.
John Davidson | President / Creative Director | Magic Feather Inc.