Actually I may have solved it!
In the Scene_Equip part, I edited Line 30...
29 @item_window1 = Window_EquipItem.new(@actor, 0)
28 @item_window2 = Window_EquipItem.new(@actor, 1)
29 @item_window3 = Window_EquipItem.new(@actor, 2)
30 @item_window4 = Window_EquipItem.new(@actor, 3)
31 @item_window5 = Window_EquipItem.new(@actor, 4)
To this:
@item_window1 = Window_EquipItem.new(@actor, 0)
@item_window2 = Window_EquipItem.new(@actor, 1)
@item_window3 = Window_EquipItem.new(@actor, 2)
@item_window4 = Window_EquipItem.new(@actor, 4)
@item_window5 = Window_EquipItem.new(@actor, 4)
It seems to be working now, letting me equip an accessory in that slot, separately from the regular accessory.
I'll do more testing to make sure it works.