The RKTs might work, depending on how long you have for it to set-up/dry. I would coat it with melted chocolate and let it dry, ant adhere it to the buttercream - (for glue) then, cover with fondant, since it will stand vertically. I am not sure about the cake, I have never tried that recipe. Good luck- Be sure to post a picture, I wish I had a chance to do a cool helmet like that. Too bad you couldn't incorporate at glowing LED light!
Are you responding to tguegirl.
Here's her post asking for help: