Some more ideas:

-Buy two helmets. Saw each in half and glue the two rears together to use. Glue the two fronts together to sell to a sucker on CL.
-Use a pursuit helmet backwards. That would be totally cool. And surely that spike out in front could only dropyour liability profile even lower.
-Looking again at the four goals you had in mind for wearing a helmet, it seems like all four would be squarely accomplished by pretending to wear a helmet.
-What about using one upside down? Could you sit on it?
-Wear a stocking hat, but call it a helmet?
-Bubble wrap. Make sure to poke holes in it for ventilation and breathing and such.

jim