My question simply put is:
Is it possible to have +Dex on an item occupying to for instance a Head slot or Neck slot or is this solely limited to the Belt slot?
Not asking about premade items, but custom items.
For a custom item,you can do whatever you want. However, from Ultimate Campaign:
Some Abilities Are Assigned to Certain Slots: Some of the magic items in the Core Rulebook are deliberately assigned to specific magic item slots for balance purposes, so that you have to make hard choices about what items to wear. In particular, the magic belts and circlets that give enhancement bonuses to ability scores are in this category—characters who want to enhance multiple physical or mental ability scores must pay extra for combination items like a belt of physical might or headband of mental prowess.
If there is a trend of all Core Rulebook items of a particular type using a particular slot (such as items that grant physical ability score bonuses being belts or items that grant movement bonuses being boots), GMs should be hesitant to allow you to move those abilities to other slots; otherwise, they ignore these deliberate restrictions by cheaply spreading out these items over unused slots.
It is possible, but they recommend against it.
Those things. Pathfinder requires no upcharge, but it also doesn't require the GM to allow "off-slot" custom items at all.
As a counterargument to UC's warning: The consequence of not allowing "off-slot" items is that the "hard choices" it mentions are rarely hard at all, and a bunch of otherwise neat items simply never get used. For some, the cure is worse than the disease.