Is it possible to design a GUI or popup window in playcanvas, or would this just amount to using z-sorting HTML divs and such? If so, what’s the suggested way to mix regular web dev with playcanvas elements?
Everything is possible 
just depends on how much work you are willing to do.
There is a pretty comprehensive answer to this on the answers site: http://answers.playcanvas.com/questions/15/how-can-i-add-a-hud-to-my-playcanvas-game
Generally we’ve been creating HTML or SVG by hand in the script code. Which isn’t great, but works. We welcome more discussion on the topic?
How should GUIs work in PlayCanvas?