I've studied a few DIY null detector designs so far, and I got to a point where I'm really wondering if a scheme like the following could work:
Take an SPDT electromagnetic relay and have it switch at something like 10 hz. Connect the relay such that its output goes into the non-inverting input of the opamp, while the input switches between the signal and ground. Have a decent ADC measure the amplified signal, get it into a microcontroller which would then report the difference between the high and low half cycles.
If this works, I would have the best of all worlds: low noise, sub-microvolt input offset and thermal EMF, low bias current (I'd use a JFET input opamp). It likely won't, which is why I'm posting it here so one of you can point out the glaring error.