A recent project at Bocoup had me searching for ways to programatically modify the user-agent string in Chrome. Searching the web yielded little evidence that this was possible, but luckily, I have a friend who works on Chrome. He clued me in to the WebRequest API, and I got to work creating my extension.
If you want to try it out, you can download it here. If you're interested in the technology, you can read about the code in an article I wrote for the Bocoup website. My hope is that this helps out people who, like me, need a programmatic solution.