Thanks for your help everybody. Using D1 seems to work fine - although now, rather than using the grove device, I've also found this project:
...which also seems to work fine on 3.3V and has the advantage of being so simple that one can solder all the components into the cable (the insulation is drying as we speak, but had it working fine with the puck.js via breadboard!).
With 3.3v I make the maximum draw through the resistors to be 0.016 mA. Maximum draw through the skin (assuming 1kOhm when wet) is 3.3mA, so very low power.
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