Having been quietly pleased about this idea for a couple of months now, it turns out I was beaten to it. I didn't realise but the QRPguys have designed a complete transceiver around the same frequency-estimation FSK technique, that they call AFP-FSK. All the best ideas!
Would be great to do a comparison of their code and mine to see how the audio frequency estimation accuracy and tracking rates compare, but I don't have the time. Either way, whilst their product is fully-featured, transmit and receive, custom PCB, and with a display and buttons, etc. mine has the benefit of being about as simple as possible (and very cheap).