Yes, you should be able to use a dedicated Web Worker linker, if what you want is running a separate GWT module as a Web Worker. You can watch the IO 2010 presentation that introduced this linker:
I don't have any personal experience with this but I would be interested in discussing how Errai can help leveraging Web Workers. If you have more ideas and/or feedback, please share it with us. Feel free to create a JIRA with a feature request if you're already thinking of something concrete. And, as always, contributions are more than welcome!