Whoops, my bad. 'Set canvas size' does work the same as 'set display resolution.' However, I just discovered a bug:
When using 'set canvas size,' with unbounded scrolling enabled, objects on a 0,0 parallax layer don't account for the new resolution - they stay where they are.
Should I post a .capx in the bugs forum?
The first time I tried 'set canvas size,' I was using letterbox integer scaling. That on top of the aforementioned bug made me think 'set canvas size' worked differently than I had expected. Gah, confusing. I will try to do more testing before posting next time :)