Yes, there are a few different pins you could use for that same purpose on the pica board.
On the ramps board, pin D12 goes from the arduino straight to the header where they are connecting that signal to the PS-ON of the power supply. There are three similar pins on the pica board connected to pins D3, D4, D5 and routed to the "SERVO/GPIO" outputs along the edge of the board. Since these are not normally used, I'd say it's a good choice. You would just need to pick on, say D3, and then set PS_ON_PIN in your configuration.h to that pin (3). Along the edge of the board, find where it says "(D3)" and the three pins above that correspond to the Signal (S), +5V (V), and Ground (G). The pin next to the "s" is the signal you want to connect to your atx power supply's PS_ON input the same way they describe. If you are using a 2 or 3-pin header to connect to the board, just ignore the other two pins because you don't need to connect them to anything.