According to the Wiki here: https://wiki.libretro.com/index.php?title=Mupen64Plus the Mupen64Plus core allows for several options that should be adjustable.
I have per-core configuration set as per this: https://www.dropbox.com/s/ybkkb6ae2etnjsr/2016-06-07.png?dl=0
But going to settings > Core ( https://www.dropbox.com/s/5t1053k6ugd3s0g/2016-06-07%20(1).png?dl=0 ) just shows this: https://www.dropbox.com/s/9rz2zk6tvi8nfbr/2016-06-07%20(2).png?dl=0
Am I doing it wrong? How can I set the mupen64plus core options?