We've done something similar with YUI and Javascript (disclaimer: I work at Y! and we eat our own dogfood). Essentially, you send an XmlHttpRequest to a background process, which collects the data and sends it back as a JSON object. The code in the browser then extracts the data from the returned JSON object and uses it to paint a DIV corresponding to the % complete.

We were even able to return a (relatively-large) logfile in the JSON object as well to get a running window into the status of the run.

This may not be close enough to what you want to work. If it is, let me know and I'll sanitise the code and post it here somewhere - probably as a separate node so it's easier to point to.