I suppose you could add a boiled lactose solution to give it a little more body and a touch of sweetness to balance out the bite, but with a RIS you'd want to be careful about ending up with way too high of an FG. If you haven't cooled and carbed it yet, I'd try that first. I had a stout a while back that my final gravity measurement sure tasted a bit more astringent than I was hoping for, but once I got it carbed up and served it turned out as a really nice dry stout. I've heard of some people trying gelatin to precipitate some of the astringent phenols, which might be worth a shot if you can find a little more info on whether or not it actually works. You may also be able to age it and help to dampen down the astringency, although I'm not sure how well dark malt astringency ages away, maybe someone else can help you there!