I have toyed with the idea myself. here's what i have come up with.
special: Rune Arms attach to the wrist there by leaving the offhand free to wield a 2h ranged weapon. as an equipment Rune Arms add 1d4 energy damage per enhancement bonus to any attack made by the wielder. As a weapon the Rune Arm can be charged as a move action each round to a maximum equal to enhancement bonus. When unleashed it launches one orb of energy per enhancement bonus (min 1) as a ranged touch attack dealing 1d6+Enhancement bonus per charge. Rune Arms may be enchanted with any none weapon enchantment. A Rune Arm may come with Fire, Cold, Electrical, Acid, Force, or Sonic energy