I've seen wallpapers in B&Q that purport to be precisely for using over woodchip - they're quite heavily embossed, though, and I'm not convinced that any of them look better than the dreaded woodchip though.
Am contemplating the same thing as I have woodchip holding some of my walls together, but most of the advice I've found on the internet says don't do it. Will be interested in what other people say here.
The Accidental Property Developer, March 2009