[dhm]

The OP made me think that what we could really use is some tricks from PERT charting and CPM. PERT charting allows more complicated relationships between tasks that simple sequential actions of GTD or the typical Gannt chart's work breakdown (also a simple hierarchy).

GTD typically defines simple sequential relationship between actions. PERT charting allows for multiple relationships between individual actions. What a PERT relationship like Finish-to-Start introduces is lag. The ability to set the next action's start or end date n days in the future based on the actual completion date of the preceding action.

Opening the while project management can of worms isn't something OmniFocus is going to want to do. But just in case there's some interest, here's a use case from my previous work environment:

There are several next actions around a code review. Reviewers get 5 working days to review and respond. A typical cycle looks something like this:

1. Prepare the review materials
2. Publish the review (action that starts clock)
3. Send reminder to reviewers at publish+4d
4. Verify all committed reviewers comments received (publish+5d)

In practice I did this through regular reviews... typically with an action between 2-3 to schedule the next actions. What would be handy is to build that into the project when it was first set up rather than building in actions to trigger the maintenance.

Maybe another way to tackle something like that is to have a self completing wait action. That is, an action added to a sequential project which rather than becoming due, simply completes itself on the due date to enable other actions that follow to become available.

Just some thoughts... hopefully my coffee deficit is not too acute at this time of the morning.