Implement Rectangular Region mode on wayland session. Unfortunatelly I don't know how to position window under wayland so it only works ax expected on one screen and without panel :(
An application cannot position itself on Wayland. What instead should be done is implement a region selector in KWin's screenshot interface that returns a cropped pixmap.
There is currently a window and color picker in KWin but no region selector.
I implemented the same hack here locally some time ago and it works perfectly fine if started with spectacle -platform xcb.
However, you'll always have to click to take the initial screenshot for the selection (but the other modes have the same issue) and with the wayland platform it's not possible to get the positioning correct over multiple monitors.
IMO clipping a rectangular selection should be implemented outside of KWin, which would make implementing stuff like drawing lines and arrows easier.