How do I add a crosshair guide to a FCPX angle editor viewer window?
It is indeed impossible to put a title over a clip inside a multi cam clip... Unless you turn that clip into a compound clip. Then you can add whatever overlay you want to it.

So you will have:


keyframed clip with crosshair overlay
inside a compound clip
inside a multicam clip


Just right click your the clip inside your multicam clip and choose New Compound Clip ( ⌥  + G )