On the (almost exclusively BA) flights I've been on, it's noted that the power jacks cannot be used for charging (it asks you not to). Somehow they manage to enforce this - I have no idea how! Two laptops I've used on planes didn't charge from a medium battery level when they were plugged in there.
120w sounds like a bit much for an at seat jack. I'd agree that you are better posting in tech log however with the profusion of different seats and so forth you may get a few different answers. That said, perhaps there's a common standard or something.
