There's a technique called inline workers, I would suggest using that.
- Create a Blob object that contains the source code for the worker
- Convert it to a "dataurl"
- Instantiate the worker with this dataurl
This is described with example code on the HTML5 rocks site in their WebWorkers tutorial. This way you could list the site as sandboxed, but since there's no need to do external requests, it should work in sandboxed mode as well.