What do you mean by viewstate encoding?
I remember one case where the user's webserver used a home-made viewstate because the viewstate was quite large and they wanted to compress it. This resulted in a hidden field in the web page called __COMPRESSEDVIEWSTATE, which of course, ANTS Load can't handle automatically. If this is the case, then part of you test script would need to include a special function to parse the viewstate out of the WebClient.HttpResponse.Content into a variable and post it back to the server. I think you may be in a similar situation?