"Don't change the HTML in incompatible ways" is probably a good general rule to live by
Not necessarily - I've only read your summary and not looked into what happened in depth, but the issue seems to have occurred from changing the CSS [and possibly js] in a non backwards compatible way. Changing the HTML didn't really matter.
but having an easy way to say "start purging all pages on $theseWikis from Squid/Varnish" would also be nice.
That sounds like something that could hurt the server kitties unless done rather slowly [at least for enwiki]...
-bawolff