I also needed this requirement and got a similar reponse to you.  In the end, like so many other things, I just found a work around. I mostly got url rewriting working in webgui with an HttpModule but then I still had one issue left that I finally gave up and just made that part of my app not using url rewriting.

It doesn't sound like you need true url rewriting like I needed. Instead you just want to remove the url parameters from a link in an email? Couldn't you just have that link point to an aspx page that strips the args, puts them into the session, and then redirects to the webgui page with no parameters?

I never got it fully working. I did get it to the point that the form would load into the browser. But it still didn't completely work. I don't remember what the issues were since this was so long ago.  I had an HttpModule with a custom stream class just for VWG that I would set as the response filter that would look for the embedded urls and resources and fix them up. I can't easily post the HttpModule since it has so much other code for the aspx part of my app. The VWG stream code also contains custom code specific to my project regarding the virtual part of the path. So what I did get working wasn't generic enough to just apply to any project.

In the end, my workaround was what I said above. I now go through an intermediate aspx page that knows how to figure out the virtual part of the url and then I just set that as session parameters and then I launch my VWG form at it's physical url.