So far, useful things found:

• there’s a minified script for making all of this happen, run it through a beautifier of your choice to be able to read it
• append #verbose to the end of the URL for JS console logging messages
• there appears to be an event listener thing going on, with what appear to be UUIDs attached to the events. I suspect they’re only format-similar though, because they must be time-dependent. Update: looks like it’s just kept in memory
• the json passed to the xkcd servers looks like so:

s {type: "comic/time", data: "{"spread":5,"image":"a901246fd70dcd0054429bf55ced123ecead832300d73dedd78857d91eaff2df.png"}", lastEventId: "c0ddcdf0-9547-11e2-8001-1c6f659cb250"}

• A helpful person on the internet is collecting them all (explainxkcd.com is doing some stuff too, but their servers are dying)
• Every 1.5min there’s an event listener update check
• There’s also a discussion on the xkcd forums
• A person on reddit made a cronjob to generate a gif of it

More as I find it

P.S.: Randall Munroe you are a hell of a nerdsniper.