Variable Current Source
There is no reason why this should not work in principle, but there are some cautions:

1) Don't use the LM393 - it's a comparator, not an opamp. Use  something like the TSB611, which is a rail-to-rail opamp which can work (just) at 36V.

2) You don't show the supplies to the opamp. Assuming it is powered from the 36V supply, the negative supply can be (for example) +12V. This reduces the stress on the device considerably, so put a 12V zener in the negative supply. The amp only draws 125uA, so you need a resistor to 36V to put the zener into its correct operating region.

3) Operating with the inputs 2.5V below the positive supply is OK - it's within the common mode range of the recommended amp - but the reference voltage could be greater (in other words, further below the positive supply) if that gives any advantage.

4) Because this circuit is referenced to the positive rail, you should use a p-channel FET, such as the ZVP3306, so that the threshold voltage is well controlled and not dependent on the load impedance.

5) Surely the value of R1 is wrong?