Strictly speaking, the rules say that composite plating can be treated as an armor item for the following purposes only:
So as far as the rules are concerned, composite plating is not an item, it just behaves like one for these very specific situations. Being designated an item familiar is not one of these.
On the other hand, it is unlikely that the authors of Eberron Campaign Setting were considering a variant from Unearthed Arcana when writing the warforged,1 and it’s absolutely certain that the authors of Unearthed Arcana did not consider the warforged when writing item familiars.2 Thus, the absence of a rule here is unsurprising, and suggests that there conceivably should be one.
Personally, I dislike the item familiar rules wholesale; the investing mechanics are clunky and (often) they put the DM in an unreasonable position since they allow a character to do things they could not ordinarily do at their level.3 This is “balanced” by the risk of losing the item—because if that happens, the things invested in the item are forever lost to that character. This is a bad situation all around, and is kind of a lose-lose for the DM: either he leaves the item alone, and the character is overpowered, or he takes it away, and the character is underpowered. Neither is great, and either can cause strife at the table.4
So I do not allow the item familiar rules at my table. Would I allow a warforged to designate their composite plating if I did allow item familiars? I’m inclined to say yes. Composite plating is often seen as a drawback,5 and frankly, the fact that the composite plating isn’t going to be separated from the warforged ever makes the item familiar rules a little more predictable, at least. But a DM who feels that risk is important might see it as a very good reason to not allow it, as the risk is nullified.
And even if they considered it, it’s quite likely that any text relating to a variant would not make it through editing anyway.
Since warforged did not yet exist.
Most notably achieve very-high skill checks.
E.g. if the item is left alone, he may have other players objecting to a character being more powerful than he should be, but if he takes it away, then he has that player complaining that his character is less powerful than he should be. Or even if the player is mature enough to accept that he gambled and lost, there could still easily be a negative impact on his enjoyment of the game, which is still not good.
It’s free, but it prevents you from simply buying better armor—you have to burn a feat to improve your armor, and that’s a lot more expensive than any of the mundane armors in the game. In a campaign that will never leave low levels, though, it can be an advantage as you can have Adamantine Body at 1st, while adamantine full plate is not affordable until 6th and not reasonable until 8th.