Another alternative would be to use email. You could write a script that sends an email formatted in a specific way. E. g. using a subject like "Re: https://tilde.club/~barnold/some-page.html" to comment on "https://tilde.club/~barnold/some-page.html". Then you could provide a script that tilde.club users can invoke like this:
comment https://tilde.club/~barnold/some-page.html <comment.txt
This script would send an email with the correct subject line and receiver address (in this case "barnold@tilde.club" as the page lives below "https://tilde.club/~barnold/") that contains the desired comment as the email body. You could then handle such emails automatically – either via sieve rules or via a cron job – to add those comments to your website. You would be entirely free to add them wherever you want. To a separate "comments.html" page. To the bottom of the page that has been commented on. Or somewhere else entirely. You could also handle comments differently depending on who sent the email in question. You could decide to "trust" comments coming from the tilde.club domain while you decide to review comments coming from other domains. Or maybe you want to keep comments coming from other domains entirely private?
Using email you would essentially provide a (very simple) standard and a single reference implementation. Other people could write other implementations. Maybe some want to add a mailto link like this one to their pages to provide better integration with the commenter's Desktop Environment or Smartphone:
mailto:barnold@tilde.club?subject=Re:%20https:%2F%2Ftilde.club%2F...
Furthermore, by relying on email as the transport medium, the "comment" script mentioned above would automatically work for all tilde.club users. Even for those that did not set up any kind of automated comment handling. Some users (you maybe?) might set up automated comment handling. Users that don't would simply receive comments in their email inbox. They could then add them to their website manually. Or maybe they decide to keep comments private and just read them. Maybe they even reply.
One last note: Commenting in this way would also work just as well for gopherholes and gemini capsules. You would simply change "https" to whatever protocol is used to serve the page in question.