My concern was basically about Apple being able to prohibit manufacturers from making hardware that they don't explicitly approve. (I don't know if they can or can't.)
Electrically it'll work, but mechanically it'll suck. Right now you hold the phone in one hand and swipe the card with the other. With this adapter, you'd need three hands: phone, reader, card. Or find a place to put the phone down while you run the operation.
For something like a card reader, you really want a device that plugs straight into the phone. If Square isn't able to build a Lightning reader just because Apple controls the port, that would be kind of irritating.
Alas, no. There are third-party NFC terminals which work with iPhones, though, so you could at least do it that way. Square has one, which talks to the receiving phone over Bluetooth.
My concern was basically about Apple being able to prohibit manufacturers from making hardware that they don't explicitly approve. (I don't know if they can or can't.)