Working on a dupe doesn’t solve the issue because later when I want to use that compound clip I created in the original project, I can’t move it to the dupe project, as far as I can tell. I’m still stuck with the same issue.
But I think I found a workaround.
First I create my own compound clip in the timeline. Then I have FCPX synchronize a couple of short clips, creating its own compound clip in the Event Browser. It doesn’t matter what clips I choose for it to sync, because then I replace the contents of that compound clip with the contents of my own compound clip using copy-paste. Now I can delete my compound clip from the timeline, but I have it in the event browser just like any other clip for when I need it.
It’s clunky, but it works!