Is it possible to use PicoPSU's with these adaptors?
Sure can. The ATX pinout is standard.
But I think you have designs on consolizing a NAOMI, right? Probably would want to skip this because it would be unsightly if you connect the PicoPSU externally. You'd probably want to drill a hole in the NAOMI case for the PicoPSU's DC jack, make your own harness (or solder to the pins on the PicoPSU), then feed the voltages to where the JST VL connectors are soldered to the NAOMI's board. Keep all of this inside the NAOMI case and looking slick.
Anyhoo, this is probably a discussion for citris3000psi's thread.