What I'd actually like is to be able to do is to set equaliser (or just loudness) values at intervals along the volume range and the player to interpolate the values inbetween. Thus giving a nice smooth loudness curve.

That's way too much work. If done properly, a plain-old loudness control should be able to handle that. My problem is that in order for the loudness to work for me in that way, I would have to increase it significantly, but that puts too much loundess in the top end. So I'm currently at a "compromise" setting on the loudness scale. It still adds a bit too much bass at maxium volume, but doesn't quite add enough bass at lower volumes. If I could just adjust those cutoff points, it would be perfect.