[DrJJWMac]

If I understand what you are trying to do, perhaps a solution is as follows:

* create a Context called Focused or Blocked or Contiguous or ...
* put all tasks that have to be done in such a manner in this context
* create a Project setup as follows:

My Project (context - Focused, due - due date, sequential)
- start it (context - Focused, due - start date)
- Intermediate Group (parallel or sequential)
-- task A (context - Focused)
-- task B (context - Focused)
- finish it (context - Focused, due - due date)

Now, when you open the Focused context, sorted by Due date, you will see exactly those tasks that you need to do in such a mode.

If you have the Mac version, you can create a custom perspective that would show just this context, sorted by due date, with either Next Action or Available views, and showing only Due. When that perspective is empty, you have no "focused" tasks to do. Otherwise, it populates with what you need, and you can set aside everything else until the work is done.

--
JJW