We're doing this now in Drupal 7. The kernel branch isn't yet. After some discussion in, I have decided to include it for now after a little refactoring of the path handling code. Basically, I concluded that it's not "wrong" to mess with the path that way because we're already doing horrible things to the path and shoving it off to a separate request attribute, and then mapping based on that. So it's no more of a drupalism than, say, path aliasing. I've left a @todo in the code to revisit the question later if we so choose.
Putting this issue here for record keeping purposes. This should resolve the following related issues:
Code will be pushed momentarily, so setting this straight to fixed.