1. I do not think you need that because nobody need to see what happens after a touch to the map "bounds" so i'd assume that you could remove the projectile?
2. If you still want to complete that, try to pre-touch hook, detect that it's touching to the sky somehow and then try to set the projectile's solidity to SOLID_NOT.
EDIT: i think better just make it MOVETYPE_NOCLIP
EDIT2: Please nevermind, i overlooked your "arch" word, that completely changed the meaning of the point.