Brilliant Energy is one of those weird enhancements that can only be enchanted on melee weapons, thrown weapons and ammunition (but not on ranged weapons itself as other enchantments can work). The combo could still work out, you just have to always buy brilliant energy ammunition.
In my crossbow build I use Earth Dreamer's ability to see through walls (earth, mud or stone, but not metal) and a phasing weapon to ignore the wall (full cover by the rules). You can even hide in the earth/wall and shoot out of it with the phasing weapon.