Cannot use SFZ and non-GM soundfonts on percussion instruments

On MuseScore 1.3 we can uncheck the "Drumset" option on the mixer and select other instruments to be played for the percussion. This makes possible to use non-GM soundfonts for percussion instruments. On MuseScore 2.0 beta 1 we cannot uncheck the "Drumset" option, so it is impossible to use non-GM soundfonts for the percussion. On the other hand, if we load a SFZ containing a bass drum sound, for example, it will not be displayed on the mixer to be assigned to the bass drum.