What I would do is keep it super simple.
I will take the normal HTML page AppGameKit exports, Change it a bit to my LIKING.
Let the Game LOADS for now... maybe with some nice animation, and once the Loading is complete...
You DO A HuGE ARROW! CLICK ON FULL SCREEN TO CONTINUE.
You can thank me later
So, you do not need to learn JS or something.. Simple y Alto.
EDIT: YOU KNOW WHAT?!?!
I JUST THOUGHT OF AN AMAZING CONCEPT THAT ShOULKD WORK FOR ALL OF US.
THE MAIN SCREEN OF THE GAME, OR, THE LOADING SCREEN.
Take a snapshot, and put it behind the AppGameKit VIEWPORT IN THE HTML5 PAGE.
THEN, ASK TO TOUCH THE FULL-SCREEN BUTTON BENEATH TO CONTINUE, SO IT WILL LOOK LIKE IT IS JUST LIKE THAT.
AND YOU CAN FAKE WRITE "*this viewport is small, so it will fit all devices" in the HTML Paragraph, "and Do not worry, See this button?
"