I found this little tidbit in ArsTechnica's review of OS X 10.0, so they deserve the credit.
Want to edit a Mac OS X system resource file in ".rsrc" format? The problem is that ResEdit only works on the resource fork, while all the resources in the system ".rsrc" files are stuffed into the data fork. Well, Mac OS X comes with a built-in method for moving resources back and forth between resource and data forks. This works with all versions of Mac OS X.
% touch ResourceFile
% cp Localized.rsrc ResourceFile/rsrc
% open -a Resedit ResourceFile
Do your thing with Resedit, save, and reverse the process...
% cp ResourceFile/rsrc Localized.rsrc
And there you have it!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031126001014824