For my next sprint car that I'm about to start building I'm leaning towards using a Bias Proportioning valve rather than a bias bar. I've not actually built a brake system like this so wondered if anyone had any pictures/knowledge/parts list of how they did it.

I'm sure I have seen that (or at least one) Spire race car use this system.

I'm assuming that I need a twin outlet MC like this:

http://www.rallydesign.co.uk/product_info.php?cPath=562_582_1311&products_id=10444

With a twin take-off banjo for the front brakes and a single banjo to the rear with a proportioning valve in the line to the rear like this:

http://www.rallydesign.co.uk/product_info.php?cPath=562_596&products_id=4108

and then the standard T-Piece at the rear of the car to split to each rear caliper and a couple of remote resevoirs on the MC (no brake light required)

Caliper wise I'll most likely use Wilwood Powerlite or Dynalite 4 pots all round with piston sizes set front to rear to give the majority of the brake bias.

I'd say that should do it. A banjo or a T piece in the front line to split L+R

A proportioning valve in the rear circuit can only reduce the pressure to the rear so the rear circuit must be outbraking the front in the first place.
Select your caliper piston diameters to achieve this.
We ran this system on our TR7V8 race car for years. We put larger diameter cylinders in the rear brakes (drums!) and used the valve to restore the correct balance for dry racing. We could then increase rear braking for wet conditions.