I have been online and found: Buckwheat hulls, also known as sobakawa hulls or sobagara husks have been used in pillows, also known as "makura" for over 600 years. In many European Countries young children are given Buckwheat Hull pillows to promote sound sleep without feeling too hot or too cold whilst sleeping. In the UK even today people are turning to buckwheat pillows to promote a good nights sleep and adopt correct sleeping positions for the back and neck.
I also found a UK company that didn't recommend heating these hulls, but they sold wheat grains and lavendar buds for heat pillows. So you can mix lavendar in with the grains - I should think that would go for rice too, if you didn't like the smell. Not sure what Buckwheat hulls smell like!