[borlak]

There is a simple way to solve the original problem and the one you mentioned. A load balancer. One that you have a good amount of control of and an API (such as Zeus, F5...)

You basically take the nodes off one at a time, wait for connections to finish, sync over code, then bring it back up. This does make some assumptions about assets -- that they are in a different location, such as a CDN or static server. If you are removing assets, you need to do this at the very end of this node-syncing process, so that any live "old nodes" aren't linking to deleted assets.

As for newly updated assets, you should be doing versioning for those anyway (even this 'symlink trick' fails when multiple application servers are involved and no shared code space).