Oh, I always imagined an addon would show something in the menu bar.
Well, an addon can create an icon in the menu bar (and in other places as well), optionally with a popup, but it doesn't have to. Regardless of that, it can also manipulate the content of a website (that's how uBlock Origin's cosmetic filter works).
How difficult would it be to make a proof of concept?
I don't know how we could identify whether the user is currently on a website supported by Keyoxide. For GitHub, Reddit or Twitter that wouldn't be difficult, but other services, such as Mastodon, are decentralised and can't be recognised just by their URL alone.
Then we must obviously figure out a way of reverse look-ups. For Mastodon, that could be easier, since the Keyoxide link must be set as profile metadata item and Mastodon itself displays a green check mark if the link is verified.
When viewing the Mastodon home timeline, the addon could, for example, simply iterate over all
.display-name__account elements and fetch the metadata information via some Mastodon server's API.
I've never done something like this before and it could easily become overwhelming, but I think it may be worth it.