This wouldn't be difficult to support in NNF. The answer would be to add another type in locked.txt "news" and add the relevant IFs (as @Martijn says) to the post/reply code to keep the file at its original date. I've been wanting the same feature to allow NNF to be used as a very lightweight blog.

I'm just going off to file the issue now. It'll be in the next release when I get to that.