It appears the magnum Ms series are isolated and have no internal system bond. I think Outback FX are, as you said, tied to batt negative (not sure if non isolated topology or not - doesnt really matter for this discussion).
So say we have two scenarios:
1) "neutral" of AC system is bonded to the equipment grounding system. Whether it is done at the inverter factory or we do it I dont think matters for this discussion and scenario. Equipment grounding system as normal everything interconnected/bonded. Say we dont (intentionally) connect to a grounding electrode. Also say that system has not incidentally become grounded by say a gas line or water line, etc. Now this would be bad because we would have the EGC system floating with one of the supply conductors and developing some degree of capacitive coupling and potential to earth which could be say a concrete basement slab we are walking on correct?
2) Ac system is ungrounded. We want to connect EGC system to earth to avoid same/similar problem in scenario 1 above (but probably less serious) correct? Now we have your basic ungrounded system but say who cares if there is a fault, it becomes a grounded system so what? Ill check it now and then and if I miss it and there is a second fault, inverter will shut down from overload and I fix the problem?
I know the laws of physics dont change when you are not utility connected, just want to take some of the issues that arise from a utility connection out of the picture to make sure I have this down completely.