After a little T&E I found which two pins are used for the power switch.
I've pointed to them in the attached picture. When I short them together the power LED comes on and the PC boots.
Does this look like a suitable place to put the TRS jack? (It does to me..)
I'm also curious if I'm correct that the green wire (from PSU) should go on the PSU side of the switch, and the black wire (that goes with green, from PSU) should go on the CPU side of the switch? The two I'm referring to are for the PSU to perform the ACPI function (power on/off).