One of the pico supplies floating around uses the LM2642 -- I assume one channel supplies 5V and the other supplies 3.3V. If they're not using that chip, it'd be something similar.
It goes without saying that you'll need to verify that every relevant part on the board can handle that kind of voltage -- 20V is a typical cut-off for some HV CMOS processes, so you may run into some issues with the switching transistors, especially.
Seems like it'd be easier to just build something from scratch instead of trying to hunt down traces on a board, but I guess to some people, that's a fun adventure! Good luck!