I am looking for any kind of solution to properly get an IIS request such as https://stackoverflow.com/% and http://bing.com/% to not display a 400 Bad Request page, but display a custom error page similar to how http://google.com/% and http://facebook.com/% do (obviously those examples are not on IIS).
I believe I have tried setting all the applicable http.sys registry settings (AllowRestrictedChars, PercentUAllowed) per http://support.microsoft.com/kb/820129 but that has not helped. Setting AllowRestrictedChars and a custom 400 page has fixed urls such as https://stackoverflow.com/%12 but not /%.