kde-plasma/plasma-pa requires the emerge of media-sound/pulseaudio even if you have apulse installed. I think a "apulse" USE flag should be added which removes this dependency.
Can you confirm that it builds without pulseaudio? Or, if not, works at runtime with apulse and w/o pulseaudio?
I tried building it with pulseaudio installed from the git first, but CMake expects pulseaudio to be installed. I then emerged pulseaudio and built plasma-pa. I then unmerged pulseaudio and linked apulse back to /usr/lib/libpulse.so.0 and it didn't work because it expects some functions in libpulse.so that are only available as incomplete stubs in apulse (only in master) and in pulseaudio 10.0. As such, plasma-pa cannot be used without pulseaudio.
Thanks for investigating this.