I second to Jeff Wilder's "double item cost (based on the slotless modifier)" idea. Basically, permanent +6 armor bonus composed of force is just like having an item which works like Bracers of Armor +6 without occupying an item slot.
Or, you and your DM may rule that such upgrade DOES occupy item body slots. You simply pay costs for Headband of Intellect +6 (36,000 gp), but your version is not headband. Maybe that is a head tattoo, a magical gem grafted on your forehead, magical runes floating around your head (like those of illumians) and such. And that feature does occupy Head slot. And it can be suppressed by dispelling.
If your version of "grafted head slot item of Intellect +6" works just like the Headband of Intellect +6, appearance and nature of it is not so important.
Except that such an item cannot be lost, stolen or sundered and thus is inherently more powerful (i.e., more expensive) than a standard item. Occupying the "body slot" only addresses part of the advantage of such a thing.