I'm shooting a music video with 9 angles. Two of them need masking effects to add another clip in a doorway to the "outside". So far the only luck I've had to get the mask is to edit the multi cam to all the angles I want, then convert it to a compound clip. Next, line up a copy of the masked clip on top of the original by opaquing the new file so I can line it up. Anyone else had luck with a quicker method? FCPX won't allow mask effects on a multicam file.