I am heavily interested in the feature OP described. I thought hiding a task did the trick until I found out that this also hides the task from the analytics.
So my proposals would be either:
A) Hiding a task only hides it from the quick select menu, not from analytics. If you want to show analytics only for a specific task, place a filter there.
B) Implement a feature to set tasks done. This will hide them from the task list and the quick select menu, but not from analytics.
To give an example why this is very important to me: We track worktime based on projects/issues. The quick select menu from just half a month of tracking already has 20 items. Things will get messy pretty soon.