I would try a few different things with this.
Regarding the Web.config file in particular, you might want to look at using local configuration files for overrides instead of leaving local changes uncommitted. (e.g. referencing an separate file that is in .hgignore). Projects I've worked on in the past did this to separate test/prod configurations from the settings for development, or vice-versa.
I don't think there is any extension which will do this for you, but you might be better off writing a quick batch or powershell script to do this workflow for you. On previous projects, I had a script which would do something similar in that it would do a pull/update/rebase to keep my changes at the tip (I was working with hg against an SVN server which made that important.)
I know I didn't answer your question directly, but I hope this helps!