So I can’t believe I missed that, good catch! 
Actually wiring the node correctly solves 90% of the problem, now it does spawn, but it’s positioning itself in a weird way: to verify this has nothing to do with my second canvas, I added it as a child to the main canvas (the default background in every new widget), and set the coordinates thusly:
It appears, but it does so at a strange offset, instead of in dead center of the canvas:
Weirdest still is that this only happens when I run it with Play In Editor- if I run it as a standalone game, the widget never appears at all, regardless of whether I’m in fullscreen mode, default window mode, or manually drag the window around to change the viewport size.