There are a few ways you might try. Perhaps the easiest is to use the the flags.
Choose a flag to designate @waitingfor. I'll say Red for this explanation. Once you have marked at least one message with the Red flag, you will find a Red Flag folder in the left window pane. It will be labeled Red, but you can Control-click the label and re-name it @waitingfor. Every message you mark with the Red flag will now show in this @waitingfor folder regardless of its account.
Hope this helps.
Robert C. Laycock (Bob)