How do I rewrite HTTPS POST requests based on form data?

Question

I have some existing functionality implemented as an ISAPI extension on IIS 6.0. Most of that functionality needs to remain in that C++ code. But all of the UI that is exposed from that extension needs to get pulled out into a new project (most likely ASP.NET MVC2 based) that is more maintainable. Nothing can change in the API. External customers need to be able to make the exact same HTTPS POST request to the URL, but the server needs to rewrite the request to something different.

My first thought was to use ISAPI Rewrite, but is it capable of inspecting an HTTPS POST for the presence of certain arguments?

i.e.

given
  HTTPS POST /blah/whatever.dll
  a=1&c=2

if form argument c = 2 then rewrite to
  HTTPS POST /NewMVCProject/mynewcontroller/mynewaction
  a=1&c=2

If this type of rewrite is not possible using ISAPI rewrite, what are my other options?