Aside from using green stuff to repair or reinforce joints, I've done a couple capes and minor tidbits. If you're planning on doing something that is protruding off a model then I'd recommend making a frame for it.
Use a paperclip to shape the upper part of the wing, flag, etc which puts the weight of the green stuff on a fixed frame which can be better secured to the model.
My dakkadakka gallery has some pathfinders that I did using green stuff and firewarrior bits, but I haven't tried making a double-sided sculpt yet. Let us know how yours goes and what works for you.