is a uA current FET or BJT circuit design possible?
Assuming you can deal with an inverted signal you could use the following (If the ideas in the comments don't work.)  The 2N7002 Vth might be a bit high, so you should look for a tiny low threshold FET.

If you need the signal to be the same polarity you can add another transistor inverter stage.  







simulate this circuit &ndash; Schematic created using CircuitLab