Well, NFC is not some emulation of serial device or such, so "transmit arbitrary data" is slightly incorrect. Also there is a specialized NFC chip in compatible devices, which strictly follows the specification, and that specification does not support HID. Which means that keyboard/drivers in quesion should follow the same specification, i.e. RFID tag reading/writing and HID emulation. You are most probably right that keyboard emulation is done by polling, can't imagine how that would affect battery life.