A shocking discovery... this works, kinda:

swayidle -w \ timeout 60 \ "/usr/libexec/xscreensaver/xscreensaver-gfx &" \ resume "killall xscreensaver-gfx" &

That will give you a minimally-functional screen saver under Wayland + XWayland.

It will be a screen saver only, it cannot lock. There does not appear to be any sensible way to combine this with swaylock such that the graphics hacks are visible while the display is locked.

(But swaylock is Wrong anyway: as I keep saying, locking should be built into the display manager, not some third-party user-space program.)

Also there's no hysteresis, a single pixel of motion unblanks. And fading doesn't work. And grabbing desktop screenshots is impossible. And and and.

Previously, previously.