I have worked a lot at this. My discussion at
http://moodle.org/mod/forum/discuss.php?d=1945
outlines what I have constructed.
This new nested question type INDIVIDUAL is now installed on one of our servers. The attached archive contains a browser shot showing how it looks like when taking a quiz where the individual questions nest the question type "embedded answers".
The question type INDIVIDUAL would fulfill your needs but unfortunately it is only implemented at our local installation and it has not been committed to Moodle core. Partly because my discussion has genererated very little attention, forcing me to rely solely on my own ideas and input from my assigners, and partly because the medical statistics department at Karolinska Institutet, on whose commission I developed these features, did not want to finance more development than necessary. (They have already financed about 10000$ worth of Moodle development, by having me implementing this and other stuff.)
Therefore, the development of individual questions preceeded only until all the demands of the statistics department were satisfied, meaning that question text editing support for individual questions was never implemented for anything but question type "embedded anwers" with the correct indiviual answers being imported from file on a very specialized format.
As a result, the existing solution must be further developed before it can be used by anyone but our teachers at medical statistics. However, as I have wanted say I have already developed a lot on this but there is still a need for more to be done before my work can be useful to everyone.