Well I tried the alpha channel quantisation idea, tweaked it to slightly smooth out the edges, and here it is:
<img src="http://www.scirra.com/images/crisp.png">
I've called the shader 'Crispify' and it's in the next build. Note the edges are crisp, but the internal colours are still linear filtered! Better yet, it's shader model 1.1, and no distance map needed.