Yes, you'd have to somehow separate the dynamic content from the static content so they could be fetched in different requests and then combined (probably via ajax). If it's just your username and karma then it's simple enough, but if the comments are displayed differently for different people then it could be bear.
I think the only way PG would try this is if it's clear that javascript, client side hacks, etc are not required -- and I think that's the case.
Aside from the header, there's only a relatively small number of variations for any given content, right? Showdead, ability to downvote, etc? So, each of these variations gets a distinct ESI URL. Like /item_$showdead_$downvote_$etc right in the internal URL, so any combination of these is a distinct URL. Only the first user to hit any particular combination would result in a request to the backend, and that could remain in cache until its content changed. No wizardry required.
Comments are displayed differently for different people.
I once posted a comment which was immediately invisible to everyone besides me - I'm guessing it was marked as spam for some reason, but left visible to me so I think I successfully posted it.
