Exporting alpha channel from FCP

The current behavior in FCP for exporting alpha channel content is a bit erratic. To export a still image to PNG or PSD and maintain the transparency, the project has to have its render settings on ProRes 4444, or no transparency. If I export a video file however, I can set the export codec to PR4x4 and the project settings can still be 422, and I get transparency. If I use Send to Compressor to create the alpha channel, it doesn't work if the project is 422. Again, as with stills, the project has to be set to PR4x4 to get an alpha channel from Compressor.