So, they added a DrawHUD event to GM:Studio, which broke views for a while in HTML5 compiles.
I finally got my HUD back to where it should be, here’s some key information:
The DrawHUD event uses the -literal- coords on the visible canvas: so the fact that I was stretching a 400×300 play-area to an 800×600 View meant that my old HUD was 300 pixels wide, but using the new method had to be 600 pixels wide. It also renders in front of everything else, so you no longer need to force your control object into the front-most layer (which wasn’t hard anyway).
In the long run, this will make it easier to throw together a HUD (you no longer need to gather View coords), just had to fiddle with it the first time.