Re: Mega Knockback in 6th
IIRC, in 5e, it was just a 'well, it doesn't really work by the rules, but we'll let it slide' application of the Megascale advantage on one's STR. I presume it would work the same in 6e, if the Megascale advantage exists.
Just to mention, I wouldn't recommend allowing it. If the person to send flying miles away doesn't have a megascaled movement power, he's out of the battle with a single move. I would question the balance.