although nigers are passive, they are definitely not the most passive you can buy. There have been many stories of how a niger ate everything or freaked out... BUT, they are relatively passive, and i think you have a 50/50 shot with a lion. A more suitable trigger would be sargassum/bluethroat/crosshatch with a lion.
A puffer with your current set up would work nicely.
You could do a yellow tang, smaller lion (radiata or antennata) bluethroat trigger and maybe an eel...