I'd shoot 5-10 rolls a year at $20 a roll, at a guess. Probably at least 50 sheets of 4x5 if it wasn't more than, say, $2 per sheet too.
I've no idea how many other folks would or how many Ilford would need. But I don't think this is exactly a black art. Simon said they could easily do it, if the demand is there. Maybe a once per year special run, or even two years - it seems to keep well enough for those periods if kept frozen.
I have 15 rolls of 120 IR820 in my freezer now - well, 14 rolls with one in a 645 Pro back - and 10 rolls of IR820Auro 35mm.