Sorry Jack, there's no easy fix here. (
IMHO).
If you're able / willing to wade into some moodle code, here's a couple of pointers off the top of my head:
(all of this obviously comes under the usual disclaimers (IIRC ...)
The quiz_newest_state table has a field newgraded, if this is set, the question has been graded.
If not, the question may have been seen, and answered (saved) but not graded. You'll need to consult entries in the quiz_states table, and examine the event field (int) - this is set depending on what action the student took that led to the creation of the field. The action the student took that led to the entry being made in the quiz_states table is called an event.
What an event id means is defined (in 1.5.3 at least) in mod/quiz/locallib.php. I'm fairly certain this is different in 1.6 code, but I don't know that off by heart yet.
If none of the events indicated a saved state, or graded state, then the student has not submitted an answer.
Perhaps you want to identify a page with unaswered questions by a different colour on the paging bar? Up to you.
Hope this helps (you or someone else)
Pete.