So, interim map generating and rendering fine. Scrolls around OK, have set up correction for position in the click detection. Floating text and other elements work fine, both under and over, which means the Custard Bar will be a doddle. Something's wrong with the node selecting though, I'll fix that this weekend, then we can move into the Node Screen. Not gonna do zooming until later.
Definitely warming to Proton SDK. Takes a while to get used to (lots of boost, ClanLib and so on) but it's good stuff.
edit: Fixed the node selection in the middle of the night while panicking about family stuff. Suffering for it the next morning, though.