#<space>header is a header in Markdown so this should not be a problem.
At least this is the format I know from Gruber’s design and the GitHub standard. I do not have a computer handy but I think Joplin does not render #something as a header either.
Ah you’re right. That’s what happens when you reply late at night.
Btw love your idea about using the API to add missing pieces of logic on top of the main app.
Another commonly requested thing one can do this way is sharing: have some kind of metadata indicating a note is shared (and maybe even a specific user/group it shared with). On the server you can have multiple API connections, one per user, and upon reading this tag send a note to the appropriate joplin instance. It’ll require some kind of conflict resolution but there might be already a library/package for that.