Straight to the Moon!

Question: Would you allow a stunt to let a Scion use Knockback Attack on a ranged weapon?

Yes! But only if the ranged weapon in question uses Strength as its damaging stat. Knockback Attack is a power that represents a Scion being so strong that when he strikes enemies, he can hurl them across the battlefield; as long as he's still using a physical attack that utilizes his strength, he should be able to pull that off. Shooting an arrow, throwing a javelin, using Flick or otherwise performing a range-based Strength attack against someone can be combined with Knockback Attack whenever the Scion wishes. Awesome stunts are encouraged.

Ranged attacks that don't use the Scion's Strength, however, like guns or crossbows, cannot benefit from Knockback Attack. The force of the attack in those cases is coming from a mechanism or the Scion's uncanny aim, not anything to do with how strong he is.