Honestly, the "positive" flavors you get from roasted barley are going to be better extracted in a mash or steep. The alcohol in bourbon is going to pull out way more tannins and astringent character than you want. I would just do this batch with bourbon only, added to taste, and then make your next recipe with more dark-kilned malts if you're not happy with the color/roastiness.
I can't be arsed to keep up this list of what's in the fermenters, but hey, check out the cool brewery I own!
twitter.com/2kidsbrewing .. facebook.com/2kidsbrewing .. 2kidsbrewing.com