Well, there's only the 3lbs of pale malt to convert, so it shouldn't be too tough. I've heard talk of people cold steeping roasted grains for stouts and the like. I don't know the details, as I've never done it myself, but maybe someone else can chime in, who uses this method? You could also consider mashing the pale malt by itself (or with the crystal) until conversion is complete or nearly complete, and then throw in the roasted grains and give them a bit more time. It will make for a slightly longer mash, but if your pH gets too low, it won't matter much as the conversion will have already taken place. Plus the lower pH may protect against some tannin extraction during sparging.
edit: well, that's what I get for taking forever to hit the "submit" button. I guess it's good we're all saying roughly the same thing.