I was thinking about capacitive-dropper power supplies, which limit the current from the 120V ac or 240V ac powerline with capacitive reactance (see Capacitive droppers and transformerless power supplies for more.). These can only supply a very small current without using very beefy capacitors and being quite hazardous. And it has to drop a lot of voltage through those capacitors before it reaches the load, if the load is something like a 5V or 3.3V electronic device.
In Can you bitbang wireless communication between AVRs? How about AM-radio energy harvesting? I concluded that a Cockcroft–Walton generator could be used as a sort of variable-ratio rectifying autotransformer, stepping up the output dc voltage from a fixed ac input level to a variable level determined by the load voltage. But if you try to use it to get a ratio of less than 1, it won't work; in the limit of low output impedance, you just get the input ac voltage at the output plus a dc offset that just keeps it from going negative.
Is there a Cockcroft–Walton-like circuit that works for stepping voltages down? Because then you could use a capacitive dropper to reduce the input voltage to, say, 48 V, limited to 30 mA, "galvanically isolated" from the input powerline by the capacitors, and then use the Notlaw–Tforckcoc circuit to step that down to something like 5 V at 280 mA, enough for a standard USB1 charger.
The way the Cockcroft–Walton circuit works, from a certain point of view, is that the diodes in its string are in parallel across the ac input (since the capacitors act somewhat like wires at ac, albeit wires with some reactance), but in series across the dc output. In this case what we want is the opposite: for the diodes to be effectively in series across the ac input while being in parallel across the dc output.
It isn't yet obvious to me how to do this (without active control circuitry, which can of course do this by switching capacitors around using MOSFETs), but I suspect there might be a way.