In T7034#165373, @gladhorn wrote:

My current thinking is:
KWin shows up as accessible application with no children.
Create an accessible window inside that application. This window has no relation to any other object but will probably make things less confusing for screen readers. It can also become the active window for screen reader purposes when interacting with KWin.

When the tab switcher is active, populate that window with a list of children, send focus events for them according to which one is currently active.