What's the most efficient way to derive negative voltage for a bench power supply?
A 24VAC transformer will give you as much as 40VDC with a light load. That's pretty marginal for an LM317 with zero output. Way too close for comfort, in my opinion. 

Anyway, if you were to use a third LM317 to regulate the input voltage down to +5VDC, you could use that 5V to power a 7660 charge pump to generate about -4.5V for your JFET current sink. 



The filtered unregulated voltage likely is too close to the maximum 40V input voltage of the 317 (actually maximum difference between in and out). Maybe you can use 317HV, but an 18V transformer would be better.

Unfortunately the LM7805 is only good for 35V max input, so you'd need a Zener in series with the input of the LM7805 if you wanted to use that part to drop the voltage for the 7660 with a 24VAC transformer.