I never knew such a simple question could get so complicated. My initial thought was that it should be "standing" (universally agreed upon) but that "him" should be "his." My rationale: "standing in the hallway" is a gerundial phrase being the object of the transitive verb "imagined." In this interpretation, "him" would be incorrect, because it is a pronoun and you need an adjective, "his" modifying "standing in the hallway." I understand, though, that it's much more common, at least in the US, to stick with "him."
However, it seems that Glitterspeck's interpretation, that "him" is the object of "imagined", and that "standing in the hallway" is a participial phrase modifying it, is also valid. I guess it goes to show how English can be so open to interpretation. Perhaps we need a god of usage to jump in and tell us how it should be.
Either way, "stood" is clearly a verb, and so can't be an object.