I've reworked it a little. The current state is the popup list will update based on player input, however there are two problems with it.
The first is that the currently displayed item on the popup list only updates if I click on the Popup List item in the Unity inspector. So if it was blank before, I have to click that to make it show in game, which can't normally be done in a game.
The second problem is that the popup list doesn't open the dropdown menu when clicked, regardless of how many items are in it. The items list starts with 0 items. It is populated only by the input. I do have 2 cameras in the scene, with the ngui ui camera being at a higher depth level than the other camera. (Main camera depth = 0, ngui camera = 1)
Any thoughts on how to resolve this?
EDIT: It may also be important to note that on Start, I am disabling the label and sprite for each popup list, then if the player hits a button, it enables them for that player, and not the others. Doing this step through NGUITools.SetActive.
Another edit. I just deleted the popup lists I had, and made a new one. That one seems to be working, so I guess I broke something along the way with the old one. I'll retry the setup with the new popup list and see what happens.
Last edit: I believe I got it all working now. Just broke something a long the way I guess, the new popup lists work fine.
Thanks for the help ArenMook