'Hyponitrite' is actually called nitrosyl, and it does exist, though it isn't common. It's pretty reactive, and not all metal nitrosyls are known. A prep for sodium nitrosyl (NaNO) is in Brauer's Handbook of Preparative Inorganic Chemistry, v.1, p.514. It is made from Na metal and NO gas in a tube furnace. Though it may be a transitory stage in the decomposition of KNO3, I somehow doubt it. I would expect to get potassium oxides and nitrogen if heated further as UnintentionalChaos suggests.