The great Bob Hoover definitely did a barrel roll with his iced tea pitcher trick. He had a glass on the panel, and poured iced tea in the glass all through the roll with one hand, while flying with the other. This requires positive G's all the way around. Or, in your description, a barrel roll.
It's at the end of this video:
http://www.youtube.com/watch?v=tOZEgKXJMCE
If I was Bob Hoover, I'd do inverted rolls in a C172. But I'm not. So I won't.
-- IFMU