At the command line, using "hg cat" (not sure about git, maybe "git show"), I can extract a given revision of a file from my repository. A common use is to save a copy of the particular file revision outside of the working directory or to a different name within the working directory.

e.g. hg cat -o outputfile.c -r revision somefile.c

Though this could probably be scripted to a "Custom Action", I think it may be useful enough to see as an option in the Log and Branch views. When one right-clicks on a file (branch view) or log entry (log view) the pop-up menu contains an option to Save that revision of the file somewhere via a standard file Save dialogue.