Hello guys, the Phaser STATES can not be used for a mini game because RenJS uses them internally to run. Basically, if you change your state to a minigame, it will be very difficult to return back to the story as it was before calling the minigame.
I think the solution of calling a new Phaser Game entirely (in a different div) from a CustomContent function is the right way. You just have to add your minigame files to the index.html, along with RenJS, and when you want to call the minigame you do it from a CustomContent function, remembering to call RenJS.resolve() once your minigame is over (and hiding your minigame div).
I'll try to build a simple example for you to play with.