Pretty sure that the adapter would work just as well (I don't know anything about the readers, but assume they are just encoding the CC info as an analog signal and listening to it over the microphone port... I wonder how easily they might be snooped via EMI).
Even if the electrical functionality doesn't change, the mechanical functionality has. With a flexible cable between, it will be far more cumbersome to hold the phone and reader, while swiping a card.
Yeah, you're right. I bet it would work anywhere you grabbed the analog signal from. You could probably even repurpose a set of bluetooth headphones for it.
