I've never seen anything like this, and even after a jolly good search (and I get what I want first time 99.9% of the time) I can't find anything that does this, so I'm pretty confident that this does not already exist.
I like it because it is a good compromise between giving users the freedom to edit their posts but prevent them from changing the context once the post has a reply.
Could it work in tandem with the built in thread edit timers too?
(Thanks peterska2 for getting this to the correct forum.)
I would also love a edit timer on my board, I know some boards are running it but I am pretty sure it is a custom thing as I could not find a hack that allowed this.