Hi!
Just an idea: What about announcing that your site is also available via onion-service by sending an x-onion HTTP response header on your HTTPS website?
For example: The clearweb site https://www.torproject.org/ could send a header like this: x-onion:http://examplefoobarbaz.onion/
Or in case you can actually provide a valid TLS certificate for your Onion: x-onion:https://examplefoobarbaz.onion/
Another idea would be to also provide the fingerprint of the to-be-expected TLS certificate. This could look like so:
x-onion:cert-sha256="1h89m/yelEy6l1poFiXZQbJ1s6BkrOquBl7Fd+0EOO0="; https://examplefoobarbaz.onion/ Similar to what is done with HPKP headers, but without pinning.
Follow up question: How could this be done with non-HTTP services? (XMPP, SMTP, etc.)
Best regards @MacLemon