Well, this is the first time in a while that I've had that old-school Linux "you'll have to hand-craft it" vibe.
Which I'm usually here for, but really not this week. Because custom keybinds appear to be disabled in this cut-down version of KDE Plasma and I'm not here to explore the exiting world of xbindkeys alternatives under Wayland, I'm just keeping a very small terminal window hanging out on the desktop with me.
Fullscreen workflow
sleep 10; grim
(followed by rapidly minimising the terminal window because I'm too lazy for any further automation)
Region select workflow
grim -g "$(slurp)
select region
enter
Thanks to @pence and @jaco for pointing me in the right directions, I appreciate you.
And also this github issue, which prompted an eyeroll but did contain all the relevant information.
https://github.com/emersion/grim/issues/29