You can install Mercurial package for Python:
For TortoiseHG-specific details I can suggest to inspect %TortoiseHg%\library.zip - it contains (some? all?) needed for TortoiseHG|Mercurial Python's modules, which list you can (try) to extend: "in Where is the Python path for TortoiseHG?" topic Steve Borho wrote:
TortoiseHg's entire python environment is contained within the library.zip that comes packaged
with it.
What I suspect you can do is add a line or two to the top of your reviewboard extension file to add your installed python path to sys.path before trying to import simplejson
Hint for adding python path to sys.path is applicable for you too
Recommendation from Convert Extension page
you'll need to use a Mercurial installed on top of a stand-alone Python, and you may also need to do something like
set HG=python c:\Python25\Scripts\hg
to override the default Win32 binaries if you have those installed also