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.