This is certainly something we need some time, but I am not going to be able to work on it for the foreseeable future. So it is certainly good if you are interested in working on this.
There is a related request MDL-280 (wow! what a small number) "Let the teacher to give one student permission to retake a quiz, even if after close date, or all attempts used up" which needs the same special-case conditions per user, so whatever database structure you create should be capable of being extended to cover that case too, even if you don't propose to do the work now.
Something like this could only go into 2.0, not a stable branch like 1.9, but you know that already.
If you look at how the quiz has been reorganised in Moodle 2.0, you may find that the new file mod/quiz/accessrules.php makes it quite easy to control what happens when the student goes to the quiz and attempts it.
The other bit is managing the user interface for teachers setting this up. That will probably be a bit more work.
If you want to go for it, probably the first step is to write a design (perhaps as a page linked from Development:Developer_notes) summarising database changes, code changes, and some UI mockups. There is a nice tool (Balsamiq) integrated with the tracker (see the Add/Edit UI Mockup link on the left of any bug. Then ask people to review it.
Quiz
Timer Modifier for Quiz 2.0?
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