I figured this one out - there's a bug somewhere in WSGEN which means that passing the classpath as a parameter isn't good enough; you actually have to have the classes on WSGEN's classpath.
The workaround is to look inside the wsgen.bat file and run it yourself with a classpath including the class you are looking for.
For example, I created a run profile in eclipse to run wsgen using my project's classpath and passing in the same parameters as the batch file would.
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