I'd like to suggest a similar tool to the PBC but for wheels rather than bikes.

Same idea in that we choose the rims, hubs, spokes and nipples of our custom wheels, though spokes may vary a little in weight according to length.

Would the idea be feasable?

Dt Swiss already has such a feature on their site. In addition to weight it will even tell you the correct spoke length based on your choice of rims, hubs and spoking pattern. The rim and hub choices are not limited to DT brand products. The spoke choices are limited to DT made spokes.

Do you mean this one http://www.dtswiss.com/spokescalc/Calculator.aspx

It has a very limited selection of rims.

I think it's a great idea. Wheels are the most imprtant thing on a bike IMO, so would love to twiddle around to see what's possible.