I imported a few clips from an FCX project into Premiere Pro. They played well.
Closing PP, I went back to FCX to find that those clips I tested in PP, are now offline.
Premiere Pro by default will inject metadata directly into the original files. You can disable this in the preferences, under the “media” tab. Uncheck “Write XMP ID To Files On Import” as well as “Enable Clip And XMP Metadata Linking”.
I did a quick test, and it looks like this will keep PP from altering your clips beyond FCX’s ability to recognize them.
Hopefully, FCX will have a more robust relinking tool in the next build.