Check out <a href="http://www.darcs.net/DarcsWiki/Tailor">tailor</a>, part of the DARCS project. It can convert between Mercurial and SVN, and has features that allow it to "maintain state" for ongoing two-way synchronization. It isn't exactly as you've described, but it is close (any the only software I am aware of that does close to what you describe).
<br><br>Disclaimer: I have only used tailor to convert some SVN repositories to Mercurial, and then abandoned the SVN ones, so I have not used the two-way sync stuff. YMMV. :)<br><br>Good luck, and let everyone know what solution you find and how it works -- I am sure there are lots of us who are interested...
<br><br>- Joe<br><br>