Script service returns 405 on VS dev server when using HttpContext.RewritePath()?

Question

I seem to encounter a very specific problem, apologies for length of question.

I have a script service physically residing at /somepath/service.asmx. I am using a url rewriting module (UrlRewritingNet v2.0) to rewrite calls from /service.asmx to my physical location. This module uses .Net's HttpContext.RewritePath() functionality.

My service exposes method dosomething, and on my page there is a script call:

$.ajax({
  type: "POST",
  url: "/service.asmx/dosomething",
  data: JSON.stringify(inputData),
  contentType: "application/json; charset=utf-8",
  dataType: "json"
});

When I run this code in IIS - it returns status code 200, but when I run this code on VS dev server - it returns 405 Method Not Allowed. Error message from stack trace is:

"The HTTP verb POST used to access path '/somepath/service.asmx/dosomething' is not allowed."

Notice that in this message the path seems to be routed correctly.

The interesting thing is - I'm almost positive this is a routing issue, because when I change my ajax call to the full url /somepath/service.asmx/dosomething - both IIS and VS dev server work fine.

Any ideas how to avoid that 405?