I ended up using the %include syntax for an hgrc file, in part because the server is a Windows box and I was not sure where to put a global hgrc file that could be accessed by the IIS server/IIS user.
I put the following at the end of my hgrc files:
%include D:\Hg\HgNotify\hgrc.notify.txt
The %include actually works out great because it allowed me to put the file in an hg repo of its own so that I can have a history of changes made.
Here's what a complete hgrc file looks like in one of my repos:
[web]
name=MyRepo
contact=hg@mydomain.com
push_ssl=False
description=MyRepo
allow_archive = zip
allow_push = *
baseurl = http://hg.mydomain.net/MyRepo
logourl = http://hg.mydomain.net/
[extensions]
hgext.notify =
progress =
rebase =
[hooks]
changegroup.notify = python:hgext.notify.hook
#Include file for common notification template
%include D:\Hg\HgNotify\hgrc.notify.txt