The dash can be updated using a hard drive on a connected machine, updating then putting it on the one without internet. As for the divx update its a no. Even downloading the file (from 360 newsgroups) and connecting the 360 hard drive to a computer and manually placing the file on the hard drive does not work.
Maybe at some point the divx update will be available to download but its not there yet!
yes - as far as I know. You only need to be logged in once for the codec to download. The dash update and the avi codec are two seperate downloads. I think it's machine specific which is why you cant download then swap hard drive to another 360 not connected.
As shb367 said. After you have the update (which you can download via MS website burn or put on usb to update 360) to play .avi you still need to download a codec the 1st time you run one. After that you do not need to be connected to live to play vids. I think they did this so banned people cant get the option to play divx. But ofcourse they worked out how to via that extender thing.