Just wanted to add: I use marshmallow root in a homemade tea or plain warm water for my GERD, it works wonders for anyone suffering from GERD or frequent heartburn. Anyway, I would've never thought of adding this to a beer, but I've read about it about it in a sacred healing beers (something like that) book I have and it seemed to be fairly common for remedies. For me, it taste too horrible... starchy, slightly bitter,maybe a little nutty but not in a good way, and bland. I could see how it might add some body to a beer like others have said but I don't see how you wouldn't avoid some funky flavors..Good luck if you go for it!
S'more beer = chocolate, lactose and dry hop with graham crackers..