Thanks for the cross-compiler help frl. Tomorrow being a bank holiday, I'll have some time to look into it properly.

Meanwhile, I've been headscratching about why the border settings exist at all, and I have a theory.
The margin_x,y values would make more sense in the fullscreen (overscan) modes than in the normal modes, where they would counter-act the tendency for pixels to be displayed off-screen. I think they may have been accidentally added to the wrong table.

Just a theory mind you.