This should be like adding specialty grains after mash, except produced wort wont be boiled, or boiled for short time.
I dont see why it shouldn't work since specialty malt is added only for taste and aroma, not for conversion (like steeping @extract brewing). Gordon Strong in his book also mentions this technique as advantage of getting most of flavor excluding possibility of harsh tastes brown malts can sometimes produce.
I would steep it and add it few minutes before turning heat off. Actually you just need to pasteurize it so any temp. above 175 will do the job. Don"t afraid of tannins except your water is too hard so you think grains wont lower pH under 6, maybe you could use bottled water for this..