Alex,
We are actively developing the web-tester right now, so your timing is great. Our current effort is to make sure all the test ads from the IAB are running well -- which is nearing completion.
Using the web-tester for automated testing with Selenium should definitely be possible. With the setup page you should be able to always control the size of the tester and the flight tab should provide a good setup. Just a note that the Fragment HTML field pre-populates from the query string. Could help.
If you can help, take a look at the issues list on the Google code project and let me know where you'd like to jump in....or raise an issue of your own.
Glad to have you on board.
Thanks,
-Nathan