The best solution for me was to install Logitech Media Server on separate raspberry pi (or another computer) and install on the LMS server shairport2 (by disaster123) plugin. Then install squeezelite (look in the Rasplex forum for how to) on your raspberry pi with Rasplex. After that your Rasplex should have a logitech squeezelite player and a airplay device that uses the shairport. Airplay is disabled in Rasplex settings, but gets airplay through squeezelite.
