Notify a user they are posting in an old thread [3.6.4]

[Caiman]

On my forum we have quite strict necromancy rules whereas most threads that haven't been posted in for 30 days or more should be left to die. But occasionally if someone does bump an older thread but makes a great contribution, we might leave it open, and in turn I just don't like the idea of a modification to automatically lock threads over a certain age, it just doesnt look very welcoming.

So what I'd like is to be able to notify users that they are posting in an old thread and show them our necro rules before they are allowed to hit submit. Ideally this would happen when they hit 'Post Reply' or 'Go Advanced', before the post is added to the thread.

So basically, I'd like a modification that adds a customisable notice to a thread older that 'XX' days which the user will definitely see before submitting their reply.

Anything like this out there already?