To the best of my knowledge there is no way to do this currently, although many have asked for it. The primary problem is that Javascript doesn't support the DOM, which is necessary for what you want to do. You might be better off using one of the windows-specific QA tools like WinRunner, etc.

That said, if you know the final form layout and list of elements you might be able to hack something w/ WWW::Mechanize by manually setting the various elements to the proper values. Check out the methods for setting form element values and submitting forms.

Good luck! Let us know what you find. IIRC Andy will be giving a testing talk at YAPC::NA next week that might be helpful.