Since playlists are required to have working Netplay Rooms, I posted a work-around that would apply here as well. I've been using it for a few months now and it's worked very well.
Why is not the core-directory of RetroArch (in the RetroPie 4.2 distribution) pre-configured to point to /opt/retropie/libretrocores where the core-files are actually installed (instead of /home/pi/.config/retroarch/cores which is empty).
Pointing to /opt/retropie/libretrocores wouldn't really work either, as that directory just contains other directories that in-turn have the libretro cores. My work-around puts a symlink of the core you're launching in a temporary location and RetroArch is then pointed to that temporary location. This allows playlists to work, but still let's RetroPie handle RetroArch the way it was designed to.