Thanks for the link, Tim. That covered some of the things I was thinking about last night. We would use this a lot, I think, so I'd volunteer to maintain it to the extent that job could be separated from Quiz maintainer, or to in any way share the workload of $maintainer as new features are added. Obviously the two are interrelated, but I'm very much aware that the more we add to Moodle, the more responsibility we have to Moodle.
As to implementation:
Given that the power of the question bank already involves many tables and therefore large joins, adding two more tables as per user profiles would have a significant drawback. The cartesian product gets a lot bigger when you multiply by two further factors.
What the link calls dynamic schema has many advantages. A wise man reminded me a few weeks ago that Moodle stands for modular, and Moodle has good support for modules adding columns to the schema, so often it makes sense to allow modules to to do things, rather than bloating the core code. MDL-40457 + MDL-40313 get us halfway there - they allow modules to add columns to be displayed and searched. MDL-XXXX would only need to address a few other places, like the "add a question" form, so modules could add complete support for custom fields.
Footnote 2 of the link, basically subclassing the database table, seems likely to be a good approach if a direct extension MDL-40457 + MDL-40313 isn't as straightforward as it appears.
To slightly extend my comments regarding "who is going to maintain it":
The demands of my users are such that I need an improved and extended Quiz, and it's likely new improvements and extensions will be needed periodically for a long time. I can think of two methods of meeting that need. I can either a) do whatever is necessary to aid, assist, and encourage $quiz_maintainer in making the needed improvements, or b) become maintainer of mod/texas_quiz/. Option B does not sound like fun. So option A it is - whatever we need to do, participating in the forum, writing new code, helping to clean up old code, whatever is needed to keep moving development forward, I'm willing to do it. I'm even willing to do something I haven't done for the last 17 years - take direction. ;)