How do I get Selenium-like capabilities while remaining as anonymous as possible during each page visit and on each form post?

I'm looking for something with web automation capabilities like Selenium that would let me conveniently fake the "browser information" that this site (for example) displays: [login to view URL] Basically, I need to change the system info available to the javascript environment on a per-request basis, to appear as a different (and genuine) user to the same web app each time I visit a page and post a form, while having javascript enabled. Having a unique IP and user agent string for each request is already not a problem. I need this to accomplish something like visiting the same page and posting the same form on the same site 1,000 times in quick succession, without giving any clue that any of the visitors are not unique users. Any hint will do, even if the solution doesn't turn out convenient. I would gladly pay for a hint that makes my job easier.