I'm pretty sure activities warranting a trunk bag are mutually exclusive with activities warranting a dropper post.
I know what you're talking about with the kicking off thing. Sometimes you find pieces of trail where you have to maintain momentum. If you come to a stop it's impossible to get going again. Then you just have to plod on foot until you find firmer ground. I've never thought of a dropper post when that happens. I doubt it will work though. I could be wrong though, so try it and report back. You won't be riding any great distances in conditions like that so you won't need the trunk bag.
If you encounter loose sections out on a longer adventure then just walk it.
Fatbikes really aren't that great on loose terrain like snow or sand as one would like to believe. Pretty good in mud though, in my experience, but I hate having my bike and myself being caked in mud.