[whpalmer4]

You could put in an action for the other side filing their brief, with a due date showing the latest date they can file. Action title might be something like:

Waiting for: Opposition brief in Bush v. Gore @due 12/9/00

The tricky part is that you would probably like to have some follow-up actions whose dates would depend on when that brief was filed, and OmniFocus unfortunately doesn't provide an automatic solution there. Your next action would have to be something that alerts you to put in appropriate due dates for your actions in response to the brief being filed.

OmniFocus does let you specify date expressions like +2d (two days), +3w (three weeks), etc. but it is counting calendar days, not court days (no big surprise there) and it immediately calculates the effective date and stores that, not the expression.

To make a feature request, use the Help->Send Feedback feature in the application.