I've seen TR and VS auraxed engineers in game, and I can find pictures of them in the googles, but not an NC engineer. Haven't even seen one in game that I am aware of. Someone post a picture of their NC Engineer in all it's auraxiumed glory, so that I may revel in your greatness.
So i've seen NC heavy's with like...neon yellow armor plates. I thought that was their auraxed armor, but if the auraxed armor is blue, what are they wearing?
Naw, that is the 'Hard light' armour cosmetic, was available during the holiday sales. It's yellow for NC, polka dot red for TR, and cyan blue for VS.
You'd probably need to drop ammo or repair a friendly MAX at least once to get the Aurax, so... maybe that's why?