[144degrees]

if i understand right ... this is what Omni does already.
I did the archive command and Omni focus removed (and archived) all completed projects, but left all the projects that had at least one action unchecked. It didn't matter what the date was . . . as long as there was an action left unchecked the entire project was left untouched.

I too was a little uncertain what would happen, but it all made sense when it was done.