To form any roll beyond a single stroke roll, you let the stick bounce so you get two strikes on the skin from one, basically wrist movement. Five stroke roll: rr ll r. The duplicated r or l is bounced, it's r l r with that bit of help.
So a variable model, in my mind, would give 2 to n, I don't know what higher number*. rrr lll etc seems a bit fancy in isolation, but let's analyze the buzz roll:
So we might as well provide the control. There he presents two bounces, rrr, as an open buzz roll, and five, rrrrrr as a closed one. *: He goes up to 16 in this lesson.
Main idea being, two discrete notes with the second of rr via a lower velocity is not it; quite like the wash of cymbals doesn't happen without it having been modeled. It's a more subtle thing than that, but the actual response, the physics of what happens to the snare drum head is not present (and then the acoustic response, + the actual snares if 'on'), and for me the reality is never really there.
And for a buzz roll, it's stringing together multiple buzz articulations which can be quite different depending on the kit piece selection. Both are more work than is ideal, and I think achievable with today's technology.