Whether it's called a "power station" or a "battery" it sounds like it's removable. Though I do wonder how much electronics you could put in it before it stops being a battery. Can you still get away with sticking "return to the manufacturer for safe disposal" or did they put a stop to it?
Considering UN38.3 is self certifiable, then back to the original question, I'd be surprised if you couldn't get away with certifying a battery pack on the basis of pre-certified cells. Apart from operating thermals, I don't see many of the original tests being invalidated by being in an enclosure so long as the method of securing was appropriately considered.