Definitive sostenuto pedal marking with playback functionality

The first time this was suggested was way back when MuseScore was at update 2.2 (by yours truly, if I remember correctly). We are now at update 4.2.1. I'd very much like a definitive Sostenuto pedal marking to be added and for MuseScore to be able to playback (correctly) said Sostenuto pedal markings. The arrangement I'm currently writing needs this very feature, so the sooner this happens, the better. Thanks!

Unless it's already a feature, in which case, where do I find it?