I didn't test, but should be possible with forward_auth (https://caddyserver.com/docs/caddyfile/directives/forward_auth)

How does it compare with https://anydice.com/ ?

What do you use in its place?

Replace cat with pv to get a progress bar for free

Unfortunately I don't have any resource for you, but I love the idea. Is there any way to follow your progress?