Could you use the activity completion and restrict access settings to make the feedback file available?
Set the assignment activity completion to "Student must submit to this activity to complete it".
Then put the feedback file in as a resource in Moodle. Set that file to restrict access to students who complete the activity (submit it). You could even keep it hidden until students meet the restriction (it's greyed out by default).
It will automatically show up for students, but won't be "sent" to them--they'd have to click on it.
I'd also indent the resource in the topic block so it appears subordinate to the activity.
I can't think of any other way to do what you want automatically. Maybe someone else can.