Can run a DAC at whatever frequency you want.For example, you can easily use this 12-bit DAC and send it data at 500 Hz, low pass filter it at 250Hz, and your 100Hz should be pretty clean if you physically separate the analog and digital signals to help keep digital noise out of the analog.http://www.digikey.com/product-detail/en/microchip-technology/MCP4921-E-P/MCP4921-E-P-ND/716280
I am (probably unrealistically!) looking to produce a +/-1V sinusoid with only nV noise in the 1-100Hz band. Is this feasible?
What's the best I can hope for?
I have no idea, but it would depend on your filtering. Do you understand what PWM is? Do you understand the limits of 8 bits? With 256 steps your quantization noise is 1/256 (before filtering) and that's not even in the same universe as 1nV out of 1V. And, that's with a normal DAC. PWM will be worse.