If a player was skilled enough to turn facing his own goal line and back heel grubber the ball into the oppositions for a onside player to ground it , you'd want to cancel the try , cos it was kicked with his heel ???
There is nothing that says hitting the ball with the heel is illegal, any more than hitting it with your head is illegal. It is the surrounding circumstances that matter. You cannot use a heel where a kick is specified eg at a penalty or a kick-off. You cannot use the heel if you release the ball forward first because the long-standing conventional exemption for a kick does not apply.