We released the requested features in 2017 R6.1.
To disable sending User-Agent header:
request.UserAgent = null;
To manipulate Accept-Encoding header:
request.AutomaticDecompression = DecompressionMethods.None;
request.Headers[HttpRequestHeader.AcceptEncoding] = "custom-value";
request variable can be created like this:
var creator = new HttpRequestCreator();
HttpRequest request = creator.Create("url");
HttpRequest request = (HttpRequest)WebRequest.Create("url");
Unfortunately, it is not possible to achieve this in current version.
However, it is very easy to implement it. It will be changed in the near future.
If you are interested in BETA version, please let us know.