I've been working on a dumb git-like and been needing to add syncing. Being a git-like it could just centralize via SSH, but i had also debated a P2P platform like Matrix or IPFS.
You use case UI-embedded Matrix interaction is especially interesting to me, because some of the UIs i plan for on this git-like are WASM based, Offline enabled PWAs.
Thanks for your work here, super interesting!
If someone wants to comment on your blog, they can write their own blog post on their blog and send a webmention. That can get linked at the bottom of your blog post with a text snippet summary.
There's no "API" beyond "curl -i -d source=URL -d target=URL WEBMENTION_ENDPOINT" in the traditional sense. Using microformats markup for better exraction is optional.
The result is a federated system of comments owned and controlled by nobody except the original author. No need to use someone's Matrix server or spin up your own. It also imposes a bit of a barrier to entry (must have your own (micro)blog), but if you don't want any random person to leave a comment that can be a feature.
Services like brid.gy turn Fediverse and Twitter comments into Webmentions as well; I've thought about using it for Fediverse comments in the past, but I don't want to host a new program or rely on a third-party service.