I solve this problem in a different way (that violates GTD thinking but it works for me!).
I like the bit of GTD that says break tasks down into small parts - it helps me try and focus on one thing at a time when I am working on that tasks but I like to see the top level task in my main ToDo. So I star the main task and filter that view on the main task.
I then create an 'Activities' task underneath this and immediately flag this as 'Hide in ToDo view' (Alt+E). This ensures that the main task remains in my Main ToDo view. I then have a 'Focus Task' tab for the big task that I am currently trying to focus on which displays in Outlook view. I zoom on the main task so that I can just see the activities that I have created under the main task. This means that as I am working on that task, I can quickly add new activiites that come to mind and tick things off as I complete them. And if I decide that I need to do a later activity now because that now makes sense, I can do so with out needing to fiddle around with task order.
At the end of the day, I mark the top level task done and record how much time I spent on it during the day. If I haven't completed everything, then I use the alternate complete option which creates a new copy of the task and drag all the completed activities under the completed copy - so I can see what I achieved in the day (can be satisfying or not, as the case may be)
As I say, its not GTD as we know it but a measure of the flexibility of MLO that one can devise other ways of working that suit your own particular style and needs.
Just thought I would share that. Now back to my focus task!!
Richard