found one - https://www.arduino.cc/en/uploads/Main/Arduino_Uno_Rev3-schematic.pdf
uses a resistor divider, 2x 10k resistors, through the op-amp... onto the gate of aaa.... FDN340P which is a..... single P-channel MOSFET. hey it's the same one already being used.
err... errr.... that's what's in the PDF of the mega2560 schematic! damn it this sort of thing pisses me off: the reference design that arduino.cc released (the actual eagle files) are hopelessly out-of-date. yes you can just cut the 2nd gate completely out of the circuit. ngggggh.
ok so that's done... but i'm still really uncertain about what (low-er cost) single-channel op-amp to put in its place?