A further concept that flows pretty naturally from the various analogies would be that a life support belt / personal forcefield harness would have two energy sources, SCUBA style: a "main air bottle" and an "emergency air bottle". You operate using the first one, which is optimized for capacity. But if it develops problems, you can gracefully stop operating by switching to the backup and scrambling to safety. And if you want to put the main one into a "secondary use" (say, priming it to explode and throwing it at an enemy vehicle), you have that option even when you also want to keep the primary use (the secondary hardware now keeps you alive and protected).
Those TAS belts do have a modular appearance, with the big box at the small of your back but with various subdivisions to the forward part. Yeah, it's probably supposed to look like a futuristic buckle at the front, a slim nonfunctional belt around the waist, and all the machinery in that box. But the clumsy artwork necessarily makes it look more substantial than that, especially when Kirk takes his off. It would be easy to argue that the front features include panels hiding the (otherwise unseen) controls; detachable power packs; and perhaps other functionalities as well.