Sometimes it is handy to be reminded of a sent mail. So a field in the compose window which reads something along "answer expected until: <date> <time>" and a copy of this mail which is specially marked in the inbox on said date.
Maybe you can add an X-Answer-Before (or whatever you want to name it) header with that info. The message view could be set to display that header and it could also be used in filters or quicksearch expressions in the sent folder... I guess it's not the same as a dedicated field but I think it would cover most of the functionality you're describing.