Energy harvesting - drain capacitor when voltage is reached, ultra low power
It seems like you want a SRC, except that the off-state leakage needs to be very low.  Triggering it will also be tricky without using some current before the threshold is reached.

A real SCR will probably have too much leakage, but you can make the equivalent of a SCR for low currents with a couple of bipolar transistors:



The question now is how to trigger this.  A resistive voltage divider from the supply would probably rob too much current.

Here is a flaky idea I just came up with but haven't tried.  Use a few diodes of the same type from the same batch as a voltage divider.  The idea being that the current thru the chain would be virtually 0 when the voltage is low, and quickly go up as the threshold is reached.  Here is the concept:



Again, I haven't tried this.  D2 thru D8 basically form a voltage divider that creates 1/7 of the voltage at the top of D8.  The current will be essentially 0 until there is a few 100 mV accross each diode.  As the voltage builds, the current will build quickly as a function of the voltage.  R1 causes a little more voltage on the base of Q1 than is accross D8 when the current thru the diode chain builds.  At that point the current that would have gone thru D8 will go into the base of Q1.  Even when tiny, that current will get amplified by Q1, which is then amplified by Q2, which is then fed back to Q1, etc, to irrevesibly turn on the SCR.

I don't know what kind of current it will take to turn on the SCR, but all you need is to have the transistors get above unity gain.  I'm guessing a &micro;A or so is probably good enough, but you would have to try this.  Datasheets don't tell you much about what transistors do at such small currents.

Here is another idea.



The SCR made from two small signal transistors still does the work, but this time it is triggered with a FET that has the appropriate gate threshold voltage.  That's going to be more flaky and unpredictable since FETs don't magically come on at some "threshold", and even if so, that will vary from part to part.

R1 is there to keep the SCR off until the FET is really starting to conduct, not just allowing normal leakage thru.  I haven't tried this either, but for a one-off it might work by selecting the appropriate FET manually.

The base drop of Q2 will probably be only in the 500 mV range when it reaches unity current gain and the SCR triggers.  With the example 2 M&Omega; resistor there, that only means a loss of 1/4 &micro;A.  Of course you'd adjust R2 to be as high as possible but still not trigger the SCR from the FET leakage current.