That’s cool. I actually wanted to do a top-down. Perhaps we can bounce ideas off each other. If you’re interested you can PM me on the forums.
As for your question I believe in that game they made individual graphics per weapon and facing: http://goldenhylian.tripod.com/l2plink.gif ← example.
While I have never done what you are seeking, I believe it would be possible by creating sockets(like you said) and just placing your additional graphics in the sockets. Having multiple animations using sockets might be difficult to synchronize though; make sure each frame is the same length and align with your characters movement.
Updating them is pretty easy. You just set which flipbook to use depending on which input is being received.