> could not find any reasonably practical way to verify the CD image PGP seal

There is no easy or obvious solution here. You can not verify anything PGP unless you can directly (!) obtain the public key from the person in question. So-called Web of Trust is a theoretical construct.

> Who wants to install an OS without being able to verify the image seal?

Well, for example, Ubuntu downloads (and checksums) were available exclusively over plain http until 2018 or so. Flavors are still http.