It's because the mirror tower beam isn't considered a projectile by the game like arrows or rocks. Because the game can't modify the values for its range like normal units, Modify Protounit won't have an effect. The same problem happens with anything that shoots lightning, like Son of Osiris.
Only way I can think of is modding, and even that has its downsides.
Another alternative is modifying the mirror towers speed to some ungodly amount, that way it could move over the map, kill a unit with a few blasts, then move back very quickly?
Shut up, I'm grasping at straws here!