Class B power amplifier using only Nch FETs
I found a circuit on Tubecad that seems to do what I want provided that the opamps have their own independent supply rails (so that the MOSFETs - or transistors in their version - can still be fully on even at really low supply voltages). They were designing an audio amp, but the circuit is going to be the same if it was re-purposed as a 2 quadrant linear power supply (i.e. it will source and sink current while trying to maintain its set output voltage)

http://www.tubecad.com/2005/January/blog0034.htm



Note. This is only the power stage, it does not include an outer control loop