The best way to get the temp up is to decoct part of the mash and add it back to the main mash. This way you do not add to the volume at all. Read up on decoction mashing. I'm too late with this suggestion for this batch though.
it might be a little dry because of the low mash temp, but i think you can add some malto dextrin if you have some. chocolate stout might be really nice if it is dried out. you might back off 10-20% on the bittering hops to help with body/sweetness. next time, put about 2 gallons of boiling water in your mash tun while your strike water is heating. when the strike water is about 14 degrees higher than your desired mash temp, empty the mash tun, then add the strike water and the grains. that should get within 1 or 2 degrees of your mash temp. if it is too high, stirring it should bring it down enough.
Well your approach would be fine too providing he had room in the mash tun and boiler.
I was not trying to get him to decoct other than to raise his temperature to the intended temperature without increasing mash volume. I just take some of the mash and heat it to 154F and let it sit to convert and then bring to the temperature needed for the infusion to arrive at 154F which Beersmith can calculate right on.