I am actually the developer of Managed Fusion Url Rewriter. The Url Rewriter doesn't need to be compiled to 4.0 to work under 4.0. You can leave it compiling under 2.0 and it will still work with the integrated pipeline on IIS and .NET 4.0. You can download the latest source from here:
http://github.com/managedfusion/managedfusion-rewriter/tree/master/Source/
Also I created a project that allows you to define your MVC routes in your rewriter config log here:
http://github.com/managedfusion/managedfusion-rewriter-contrib/tree/master/Source/
And can read more about the project here:
http://coderjournal.com/2010/03/editable-mvc-routes-apache-style/
As for the 500 errors, I am probably going to need a little more information, can you please update your post to include maybe a log from the rewriter enabled by adding the following to the top of your rewriter:
RewriteLog "log.txt"
RewriteLogLevel 9
This should help us better diagnose where the 500 error is coming from. But given all of have written above, I have concerns that you may not understand the difference between routes and rewriting, which many developers don't because they are sort of similar in what they accomplish. Which is why I wrote the following post:
http://coderjournal.com/2010/03/difference-between-routing-rewriting/
If you want to work off line from StackOverflow back on the forum I would be happy to help you.
http://urlrewriter.codeplex.com/Thread/View.aspx?ThreadId=228348