You know, the human hair thing really makes sense to me. My darling gram taught me to take a pin, needle, safety pin etc and rub it thru my hair if it wouldn't easily go thru fabric. I tried it and it worked like magic! Still do it to this day, works like a charm. So it kind of makes sense to use it in pincushions. :-)
Regarding the sand; I also have heard you must bake the sand on a very low temperature. I just use ordinary polyfill.