You cannot force people to automatically enter entries in their .hg/hgrc when they clone - this will make it possible for someone to put rm -rf ~ into the commit commit, and that would not be a chip for former cloners.
What you can do is include a file called mandatory.hgrc outside the 'hg directory, and then tell cloners to put this line in their .hg/hgrc :
%include ../mandatory.hgrc
Although it still shows you a lot of trust.
If you manage the machines, as in many corporate settings, you can use your tools for package / deployment / update (we use a puppet) so that you put everything you want into their /etc/mercurial/hgrc .
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