When I last harassed Crawshaw about this and we discussed bits, he submitted https://github.com/C2SP/C2SP/blob/main/well-known-ssh-hosts....
Unfortunately neither of us has taken time (AFAIK) to go back and implement it anywhere.
Honestly at this point I'd be kind of happy if we could just use an x.509 cert from a webpki acme provider in the sshd and be done with it, for the host identity part.