Just discovered Clippings in OF. That does pretty much all I need. I can add a "waiting" context to each clip I send over to OF from mail. No rules required.
It's not onerous, however, to do this manually. As you've discovered, just switch to Sent Mail folder in Mail, select your sent message, clip it to OF, and later adjust the task title and context as desired in OF. If you configure Clippings to open QuickEntry, rather than send directly to the OF Inbox, you can streamline some of those steps.
If you come across a solution for the script, please return and share it!