have you considered a small addition of flaked rye or flaked barley? just a couple % of the grist shouldn't really lend any flavor to speak up while bumping up the body a bit.
I know my session stout is packed with body at ~30% rye.
I took the challenge to brew this, and used 30% flaked rye/wheat. I'm bottling this weekend, so will update then with tasting notes.