I agree wholeheartedly with your desire to keep the camcorder intact. This is as a result of a few attempts to repair VCRs----which are mechanically much larger. The idea of loosening a screw on the camcorder and hearing "click, click, sproing!" gives me the shivers! Replacing the whirring noise of the motors in the submarine with a telemtry stream seems very sensible.
I've managed to get 230KB async from a UART through a kilometer of intercom wire (simulating an oceanographic cable) by putting out full cycles of pseudo sine waves for 1 bits and nothing at all for zero bits. The signals were transformer coupled for impedance matching and DC isolated so the cable could be used for HV power.
I think a similar approach would work on an audio recorder---but probably at a lower baud rate. I used an SX chip from UBICOM as a modulator/demodulator. For lower baud rates, a PIC would probably do the job with less power dissipation.
Mark Borgerson