Using a Hop Back between mash and boil for dark grains.

I live in portland, and our water here is close to pilsner water, or with very little mineral content, almost like distilled water. What would happen if I separately mashed my base pale malts with this water for about one hour in a recirculating direct-fired-mash tun, and then added in line during sparging a container similar to a hop back, but filled with the darker roasted grains. I usually sparge for about 1.5-2 hours, so the darker grains would be exposed that long as the sparge water percolates throught the dark grains into the boil kettle. Is this a way around having to treat your water chemistry?

You could always throw your dark grains in a grain sack and toss them in your pot as you sparge the wort on top. This will add some of the color you are looking for. I find putting a grain sack in the pot as you sparge to be particularly effective when partigyle brewing for the second runnings.