Yes, you'd get a bit of a 50/60 Hz buzz when implementing the technique I suggested at face value, but wouldn't the square and noise channels overwhelm it in practice? Or could it do the DC correction buzz in sub-frame, sub-DAC-quantum steps?

I thought of just highpassing the DAC, but then you wouldn't get the nonlinearity in the signal, which some games use to control the volume of the triangle and noise channels.