Water chemistry/steeping dark grains seperately

I'm getting ready to brew a Russian Imperial Stout, which is going to have quite a bit of roasted barley and other dark specialty grains. I've done several AG batches now, and I always use Palmer's spreadsheet to figure out water chemistry adjustments. That seems to work well for me, but my local water profile has such low Residual Alkalinity that I have to add chalk or baking soda to the mash for just about every recipe. For a really dark beer like an RIS, what if I steep the roasted barley seperately, and just mash the lighter grains and base malt? That way I wouldn't have to make such extreme adjustments to my water. Has anyone ever tried this?

You could always add your darker grains at vorlauf. I've done this about 5-6 times lately and I have enjoyed the results. Gord Strong talks about this in his new book.

This is a workable method of avoiding too low a mash pH.

Steep the grains in your sparge water. This'll also increase the acidity of the sparge which is beneficial as if the sparge isn't acidic enough bad things are extracted (can't remember what)

...not that it'd be an issue unless you're doing a ridiculously huge sparge.