Just happened to find a solution to my own question:
Instead of adding the source folder to the classpath of the Codeserver run config as in Daniel's instructions, I added this source folder as part of the command line arguments using the -src argument (see here for more info).
So the arguments string for the Codeserver launch config should look like:
-bindAddress <codeserver-ip-address> -src <gwt-source-path> <gwt-module-name>