By default 'rbt patch' uses GNU patch with a prefix value of 0. With Mercurial, patches often fail to apply because they're fetched from Review Board with the git-style prefixes 'a/' and 'b/'. Also, on Windows the binaries available are very outdated and the only way to get a new version is to install Cygwin.
Avoid these problems by overriding apply_patch() and running 'hg patch' instead.