redirect for specifiic URI with offsite web servers

That would look something like:

if ( (http_host == "mydomain.com") and (http_uri == "/a/b") ) {
  redirect to http://redirectdomain.com/b
}

Now, assuming you want to take whatever /b is from the initial request and pass it on to the new server, then you'd have to do something a little more involved:

if ( (http_host ends_with "mydomain.com") and (http_uri == "/a/b") ) {
  redirect to "http://redirectdomain.com" + getfield(http_uri, '/', 3)
}

I think this is pretty close to what you're looking for. Let me know if you have further questions.

-Colin