Thanks for the link Jon, I love reading things like that.
I'm still in two minds about this one. 3 or 4 IC's (depending whether I use a quad) is do-able of course, but even after removing the switching components there are 5 transistors which will need to go in there too, and I don't think that would be do-able by my usual criteria.
But it is a nice effect and a worthwhile addition so this may be one of those that I make up with a 1590BB in mind, I'll have a think about it. But it won't be any time in the very near future, I've got tons to go at already.