Did I ever say a mechanism to "run arbitrary code"?  I do not believe I did, nor did I even hint at running arbitrary code. What I want is a mechanism to run another hg command, in fact a specific hg command. Not "arbitrary code".

Which boils down to arbitrary code :-) Your type of proposal has rejected repeatedly and no one has proposed a safe way to do it. If you feel this is easy, then by all means submit a patch to mercurial. You might as well train people to use 'hg pull -u' and have ./configure add the hook but Satish maintains that this doesn't solve all problems (which he is right) so it's useless (which I disagree with). Oh well. What do you propose, Barry?