Hi Tom,
In short, some are possible. In the longer term, we would like to offer more, but for now we are restricting to things that are not destructive or compromise security, until we can find a way to securely use the feature. (You could imagine a case where a URL is on a web site, and it requests your note content, and sends it to some server.)
Right now, you can mark a note done, and append to a note. You can’t export or get the contents, for the security reasons mentioned.
The complete list is here: X-callback-url Support and Reference
See append-to-note. You can add text, but also control other settings like on-the-agenda.
Kind regards,
Drew