You can set up an allow based on key words within the content groups, however, there isn't a way to specify where this is being taken from. So it may not work since it's in the subject line.
If there is anything else unique you might be able to use that though... Under Email Defense, go to Content, then Content groups. When you set up a new word list you can set an action of allow. Be sure to test this first to make sure that it doesn't incorrectly flag anything.
Thanks!