In theory we can support a 'clipping' of any geometry, but the difficult part is letting you choose what that geometry is. There is a lot of effort involved there while you can much easier achieve it with a simple background image, or an opacity mask if it should cut off the contents as well.
After some test I see long ticker (as you do in ATVO Theme NEW) take a lot of CPU "ressource" if the referent SubWidget use 1 or different backgroung picture. And sometime ATVO begin to go slowly. It seems that is not the problem if you use only THEME EDITOR options to create your referent SubWidget. (Perhaps I'm wrong)
I try with only OPACITY MASK to do specific geometry, and it seems OK.
I just ask, cause I try to realise some new Theme and see we can do a lot with only Display/Border Thickness/Corner Radius .... and background linear or radial gradiant .... And I think during those test with Chamfer, we can extand possibilities.
So it is only a thinking and as you say if you need to do a lot of work for that, their is no interest to do that, I agree.