Battle caster has no requirement of actual proficiency with heavier armor types. Still doesn't mean it is that great of an option.
I'm AFB right now, but I'll take your word for it. I might have it confused with something else anyway.
Mithril Body isn't bad for a warlock though. It's still light armor, and you can thus ignore the ASF without the need of Battle Caster. Basically you trade +3 AC for -2 ACP.
I toyed with suggesting Mithril Body. If you're going warlock and warforged, than this might be a good way to boost your AC. I hate spending a feat to do this, but warlocks don't really have much in the way of AC boosting, and warforged can't wear armor.