How much value might such an NFT have? And how many such NFT’s would have a marketplace that might come up with a non-zero price for that value?

For 99.999% of such NFT’s, probably none.

But for a small few, yeah, it might be significant. You might be able to own the first post/comment by Famous Person X.

Hell, didn’t somebody buy Jack Dorsey’s first tweet? That was an odd one-off, but it’s a precedent.