Optimized Media: Enable for Offline Reference Clips

I often get reference movies from editors as compressed H264 files, and when doing conforms using those files as Offline Reference Clips, the scrubbing/playback performance is sluggish.

I would like to be able to apply "Generate Optimized Media" to Offline Reference Clips, just like any other imported clip. But this does not appear to work. The option is available when selecting an Offline Reference clip, but selecting it does nothing.