Proxy only certain footage?

I have a library containing some 4K XAVCi footage and some HD H.264 footage. The XAVCi is making my Mac Pro groan. Ideally I'd like to make proxies of the XAVCi but not of the H264. But FCPX seems to need ALL footage in a project to be proxied to view any proxies in the timeline without other footage showing as Missing File/Proxy.

I was wondering if anyone knows if it would be technically viable for FCPX to enable some kind of combination-view of the timeline, combining the proxies with Original/Optimized footage. I ask this to know if there's any point sending in a feature request for this to Apple

It never hurts to request a feature, even if it's one that only a few people feel would be helpful. I actually agree that being able to view both proxy material and original material in a project timeline could be helpful, even though I rarely use proxy media. At present, as you have found out, toggling proxy on reveals only proxy media and your other clips show up as "Missing Proxy".