While this feature has not been implemented to date, I made some custom macros to help my workflow for now. Each macro just renames a clip to a predefined name. So with Cmd+1 through Cmd-5, I can rename a clip *, **, ***, ****, or *****. To flag a clip, I set Cmd+P to rename the clip !P (I don't know, it looks like a flag to me. Plus the ! grabs my attention and adds a sense of urgency.). To reject a clip, I made a macro that renames the clip to /x, and then mutes it.
This sort of works for now, but it's not the greatest system. Rating or flagging a clip this way will lose the original clip name, which is confusing. And I can't both rate and flag the same clip at the same time. If this feature is properly implemented, it would also open the door for other functions like "Select all Rated 5" or "Delete rejected". I'll eagerly await this feature, but for now, I have some of my workflow back.