Actually it's super-easy, barely an inconvenience.

Dedicate a portion of your site to notifications. Allocate the URL there with a blank page that has a meta 404 tag or something.

When blog post is live, replace the meta 404 with a meta redirect to the real permalink.

You can do an all-manual URL shortener for QR codes the same way. That means you can also have a QR code for this kind of subscription, which is cool

How does the user agent know this is 2 of 4 or 2 of 2

Anyways the convention isn’t too hard it’s the adoption by authors and software that will be tricky

Ah yeah, simple manual methods are tight.