Yes. I cold steep my dark grains 24 hours or more before I brew and then add the liquid from the cold steep into my boil during the last 10-15 minutes just to kill the lacto and whatever else was living on the grain.
But you're brewing tonight, so you don't have enough time to cold steep. I like your idea of tossing them in the mash after vorlauf, or really anytime after conversion is complete. The dark grains had affect your mash PH so you remove them from the mashing process. Good idea.
Let us know if you get a less astringent porter.