Hi @sietse,
If you don’t need fine-grained control then it might be sufficient to set up some custom MAVLink commands in QGC, which would at least allow you to set the lights to some pre-defined brightness levels, and potentially several if you’re ok with adding on-screen buttons for them or sacrificing an extra joystick button or few 
Incremental control unfortunately doesn’t have an existing MAVLink command, so would best be handled by a firmware modification or a Lua script.