Here is a very simple circuit that must have been done before but I don't recall ever seeing published. I call it an emitter coupled latch. I guess it's an emitter coupled schmitt trigger taken to extreme.
The lower sketch shows how I used it in an inrush limiter to monitor when a big supply capacitor gets charged to within a volt of final before enabling the smpsu main load. The mosfet which bypasses the inrush limit resistor is activated by an aux output of the smpsu (in my case the RCD snubber network on the main switch transistor was handy). The residual inrush surge is much lower and easily managed by inductance and wiring resistances.
I was dreaming up ever nastier hairball circuits involving comparators and bucket loads of parts before I came up with this circuit. It now seems so obvious, have any of you folk seen this before?
piglet