I might be a little off-target from what Mark is asking, but I do want to add some ideas that might help.
If the question is "Can a set of variables, such as x, y, z, be used across a group of questions?" the answer might be yes. I say this because this is what I am doing with the Calculated Multichoice question type.
For example, if one wants to use two randomly, let's say x and y, which might take on the values 4 and 5, and then ask: For a rectangle with sides of 4 and 5, 1) What is it's area?, 2) What is it's perimeter, and 3) What is it's maximum distance between corners?, this can be done. All questions would use the values 4 and 5.
There are a few qualifiers for what I say.
1) Calculated Multichoice doesn't create the random variables on the fly, you create this "set" of randomly generated variables when you create the question.
2) The questions that are going to use the same set must be in the same question category.
Also, the Calculated Multichoice question type takes some study, like 8 hours of experimenting, to start appreciating what it can do. (So Jean-Michel, I can appreciate what you are saying about needing a block of time to get back into an issue.)
Having said this, I do want to add that I find this Calculated Multichoice question type to be one of the most powerful "Moodle built-in, supported" question type, meaning that it is not an add-in.
Well, hope some of this is helpful.
(Actually, I am thinking about giving a presentation at one of this year's MoodleMoots on this topic. Might anyone find this useful?)