[curt.clifton]

How 'bout a script based solution? A Complete and Wait For Response script could duplicate the item, check-off the original, and assign the duplicate to the waiting for context, perhaps with an optional future start date. You could add this to the toolbar for easy mousing.

I might be persuaded to bang this together if I thought someone would find it useful. (No promises; this weekend is booked solid and next week is looking scary.)

(In my ideal world, I would be able to write a script that is automatically executed whenever an item is checked off. Then I could check whether the completed action is in a particular context, say Email, and automatically create a related Waiting For item. Perhaps in OmniFocus 3.0.)