It'd be nice to see these "early tests".

You can easily try it yourself.

python -m SimpleHTTPServer 8080

creates a webserver serving the current directory, you can then create an email linking to a file in that directory and observe when it gets queried.

Easily is a bit of a stretch, because most users are on NAT setups and they would need to go into their router settings and know how to set them up to allow the external request to get through. So, yeah easily if (a big if) you know how to do that, or if (another big if) you are on a machine that is directly visible on teh interwebs.

Easy to do with ngrok.

That's not the point though. The point is most users don't have the knowhow.