First off, post this on spout. Secondly, be more specific. Explain how the game decides if you should have a weapon on your back. If it's equipped, it would be in your hand not your back. If it's not, say you had 2 iron swords and a gold one. Which one gets thrown on your back? Also, animations won't happen, even with spout.
No, there isn't a plugin like this. It wouldn't even be possible to do without Spout, which is what everyone else has said.
There is no need to be rude, I was simply answering your question, which you didn't seem to be able to deduct from the other answers.
He was answering your "other unanswered question" as well. There isn't a plugin alike this yet. So it would be best to request it on spout forums.