Ok, this has been on my mind, so I did a small test today.
I'll post up the autopsy picture as it gives a better view of the whole hand.
What started me thinking was - if I was going to draw on my palm or someone else's palm, I would hold my or their palm flat. It makes for better movement of the pen, than drawing on soft fleshy skin. I've seen other people suggest it could be a smile with two downward dashes for eyes, a smiley face. So I drew a similar style smiley face on my flattened out palm. When I then held my hand loosely, almost but not quite cupped, with the fingers curled inwards like the photo, it became more heart shaped, like the photo.
I also came to the conclusion that this was probably drawn not by JonBenet but by someone else, sitting at JonBenet's left side. This is because if she were drawing it herself with her right hand, I feel the smile would be as she would be looking at and holding her own arm and hand out comfortably - more central or probably towards the side of her little finger - rather than on the side of the base of her thumb (mount of venus). It is too angled away from her own line of sight or perspective.
So, a smiley face drawn before her hand was tightened by rigor mortis, and drawn by someone else.