Kinda-not-really? Wayland has wl_region objects and wl_surface_set_input_region but wl_region only has axis-aligned rectangles. you'd end up approximating the circle (or whatever shape) as a union of horizontal rect slices, rather than a pixmap. you can't just hand over a pixmap mask, you have to decompose the shape into rects yourself.
Why ? I'm sick of square windows. I want disc windows. And instead of scrolling them, i want to rotate them. /s
Fixing bugs is hard. Better focus on the aesthetics.