Regarding embedding and attaching weapons as warforged components, p. 114 of the Eberron Player's guide
Weapons can be embedded or attached components. Two-handed weapons cannot be attached or embedded. A one-handed ranged or melee weapon can be attached to a hand, although you can still have only one weapon per hand, regardless of whether the weapon is held normally or attached. An attached weapon occupies your hand, and you must remove it to free the hand. A one-handed weapon that has the off-hand or light thrown property can be embedded.
Regarding double weapons, p. 10 of the Adventurer's Vault
Wielding a double weapon is like wielding a weapon in each hand.
Additionally, almost every (if not every) double weapon in the game has the "off-hand" property.
Could a warforged attach, or even embed, a double weapon to one or both of his arms? If he can embed the item, could he theoretically retract the entirety of it within his arm? It's silly with most double weapons I could think of, but specifically I was thinking about the spiked chain, which gains the double weapon and off hand properties with the spiked chain training feat, p. 11 of Dragon Issue 372
You gain proficiency with the spiked chain. You can treat the spiked chain as a double weapon and a light blade. As a double weapon, the spiked chain deals 2d4/2d4 damage, and it gains the off-hand property. For more on double weapons, see Adventurer’s Vault, page 10.
Could a warforged embed or attach one or both ends of a spiked chain, if he takes that feat? Could he embed the entirety of the chain in one of his arms, or just the end?
I'm asking this because of a theoretical Warforged Melee Warlock I've been coming up with, who uses a spiked chain enchanted as a pact blade as his weapon to use Eldritch Strike, and use both ends of the pact blade spike chain for dual implement spellcaster. Could my warforged embed part of the chain in one arm and, while not use it to melee attack, still use it to cast spells as an implement? Regarding implements and embedding within warforged, from the WotC article Playing Warforged:
rods and wands can be embedded and hidden in your arm and still function, leaving your hands free for other tasks.
Now, obviously it does not say "pact blades" or "weapons functioning as an implement", but these are one-handed implements much like wands or rods. Is there a way of having a light blade function as an implement while still embedded within a warforged's arm?