My workaround so far;
Install the Mercurial command line tools (Mercurial 1.2.1, not TortoiseHg 0.7.5). This doesn't install the misbehaving shell extension. You get to use Mercurial, but only over command line. Maybe a ST plugin would be advantageous here, allowing you to do pushes, pulls, merges and commits? I've already done a rudimentary SVN plugin which I used regularly, so something should be possible.
Once you've installed the Mercurial tools, you should be basically good to go, except that SSH access needs to be configured. Where TortoiseHg installs TortoisePlink.exe, the Mercurial install does not. You can get TortoisePlink by installing TortoiseSVN, and then adding this to your mercurial.ini file;
; secure shell.
ssh="C:\Program Files\TortoiseSVN\bin\tortoiseplink" -ssh
That should be enough to allow Hg and ST to coexist without too much grief.