Correct assumption on the P360's. When you just have the +5V and GND wires connected on a P360, the L/R/U/D tabs measure about +4V. When you connect the directionals to the button inputs on an I-PAC, they still measure about +4V. When an opto is blocked (ie, moving the joystick "up"), the UP terminal on the I-PAC goes to ground, which is what is supposed to happen.
Based on what I think you are asking, I say you are okay. Best to confirm with Andy or Randy, though, just to be sure.