I have a similar need for timeslots for pickup(delivery), example:
if a customer places an order

• before 12:00 ->pick up from 14:00
• between 12:00 and 14:00 -> from 18:00
• after 14:00 -> next morning at 9:00

calculation could be done in a calculated CCK (provided not too many slots)
maybe a custom table of some sort that holds the slots
a workflow-ng condition like you suggested, Lyle, but I don't know (yet) how to use the return $pickuptime value from the 'Execute custom PHP code' - if that's the way to go at all?
Anyway, then I would have to get the returned value to show up on a custom checkout pane: 'You can pick up your order at 18:00'