Inventory is only avalible while being on foot.
In my opinion it should also be avalible while being inside some of the vehicles (Ifrit, Hunter, KA-60, etc.).
But it shouldn't be avalible in all of them (quadbike, AH-9, MH-9, bycicles etc.) due to logical reasons :)

Inventory access should also depend on your position in the vehicle. A driver/pilot/gunner shouldn't be able to access inventory while other passangers that are just sitting inside as a transport group should be able to access it.