In the end, I've dropped using the NuGet package, and simply process the captcha's using the code below, binding the recaptcha fields in the controller method.
public bool ProcessCaptcha(string recaptcha_challenge_field, string recaptcha_response_field)
{
const string verifyUrl = "http://www.google.com/recaptcha/api/verify";
var res = true;
var ip = Request.UserHostAddress;
if (ip == "::1") ip = "127.0.0.1";
var myParameters = string.Format("privatekey={0}&remoteip={1}&challenge={2}&response={3}", Config.CaptchPriv, ip, recaptcha_challenge_field, recaptcha_response_field);
using (WebClient wc = new WebClient())
{
wc.Headers[HttpRequestHeader.ContentType] = "application/x-www-form-urlencoded";
string HtmlResult = wc.UploadString(verifyUrl, myParameters);
var split = HtmlResult.Split('\n');
if (split[0] == "false") res = false;
}
return res;
}
With this in place, I split my original controller method into a Async/Completed pair, and wrapped the work it does in Task.Factory.StartNew(() => { ... }), following the pattern outlined here http://www.deanhume.com/Home/BlogPost/mvc-asynchronous-controller---the-basics/67 which seems to work perfectly.