DaveM.
I'm not entirely sure completely understand what you are asking, but I'll take a stab at what I think it is...
Regardless of the tool you use to build the wf (VS or SPD), there needs to be indicator to the system that the task is complete. By default, this is a user action - clicking a button on a task assignment form, typically. However, there is no reason of which I am aware that this can't be done some other way - including programmatically. With that said, I'm not sure of the use of such a set up. I can think of a few situations where it would be useful (such as where the completion of the task itself is a trappable event and it would be useful to have that be the signal that the task is completed as opposed to forcing the user to go back to the form and click a button). However, to my mind those are the exception situations and not the rule.
Without knowing more details on what you are looking for, I think that you'll need to do this in VS. If your needs are light, you might be able to cobble it together in SPD but I think it would be fragile.
Hope that helps.
Dave