You could probably hack the MailAction.applescript inside the OmniFocus app bundle to do this. But I'm not sure that's the best course of action.
The '@' symbol in particular is what OmniFocus uses to find a context in the message, so that if you sent an email "Pick up cat food @errands" it would create an action "Pick up cat food" and set its context to "errands".
Here's some more detail:
http://www.macsparky.com/blog/2010/1...th-e-mail.html