Looking at the input power available and what Dave said about the architecture:
With the isolated DC-DC conversion, linear regulator in tandem with variable voltage DC-DC boost preregulator and LED displays, and output side power rail for powering a microcontroller which would also have to magically appear at the isolated output side where the microcontroller would be, and about 4.2V/500mA worst case input power due to USB cable voltage drop, my estimates come to at best 1W reliably usable for output.
That is 3.3V/303mA, 5V/200mA, 10V/100mA, 12V/83mA, 15V/66mA, 20V/50mA.
If you want more you would probably have to drop something.
Some power could be saved on the LED displays. Assuming multiplexed digit mode, up to 8 segments lit with 20mA each at any time, and classic resistor current limiting on these from 5V power rail, LED displays alone can take up to 0.8W. This could be lowered to about 0.4W if powered from a low voltage supply rail right above the LED voltage drop + digit switch drop. You could also reduce the current/pwm on ratio at the cost of reduced brightness. And get some high efficiency LED displays that need much less than 20mA to light up probably at higher price.
Drop the linear regulator and leave just isolated variable DC-DC with an output LC filter for about 20% more power in exchange for a bit of output ripple.
Drop LED displays and replace with LCD with efficient single LED or no backlight for another 10% or so.
Maybe you could get down to 0.5W overhead/1.5W usable supply for
3.3V/454mA, 5V/300mA, 10V/150mA, 12V/125mA, 15V/100mA, 20V/75mA.
Not sure how much lower you can get, with classic optocouplers for isolation probably not as these include power hungry IR LED that will easily want 5mA, 6.5mA if you want to leave slack for LED burnout over time... Maybe with some fancy inductive type of isolators? I have even seen an isolated op-amp with these somewhere...
I can't wait to see how efficient will Dave get with this
If he changes his mind and actually does use some USB talking chip, a great feature to add would be an isolated built in logic level RS-232 port for debugging the powered device