Hi again calculus_cuthbert,
I've done this before using diode connected NMOS and PMOS transistors which you can bias with DC current sources at the top and bottom. It works pretty well, the only drawback is that the voltage is not process compensated, so if you want to minimize variation keep in mind that you should use long transistors. Now, if that is not good enough then you will have to include some sort of control to adjust the bias current through the transistors to get the desired voltage.
Hope this helps,
diemilio