I'm noticing the same behavior.  I'm trying to do the same thing.

I think the secret is in the regex expression in your FriendlyUrls section of Host...Host Settings.

I have a custom error handler which I am using to redirect to other pages depending on the incoming url (query string).  Here's my Match:

.*/Site/(.*)/default.aspx

and my Replace with:

~/SmartErrorPage.aspx?aspxerrorpath=$1

The problem is that when a non-existing page is requested, http://localhost/blah/blah/blah/blah/default.aspx, it redirects to the error page but I'm not able to get the aspxerrorpath query string variable.  Still working on it, I'm tweaking my regex to get a better result and if that doesn't work, I'll modify my custom error page somehow.