dynamic formats with high/variable number of iterations

I'm sure this has been discussed in the past, but I don't find that discussion anywhere.

If a format has a very high, but fixed iteration count, you could still create a dynamic format, even if it is somewhat cumbersome.
But with a variable iteration count, you are lost.
How hard would it be to implement some kind of variable iteration count?
Lets say, a new flag that needs to be set, so that $$I1000 gets interpreted as iteration count 1000?
May be you need some DO ... ENDDO logic, or even handling of more than one iteration count.
In that case, I'd say: use the forst $I for the first (or outer) loop, etc.

Do I remember wrong? Where has this been discussed in the past?
Should this be discussed on john-users for additional input?