That's an interesting question. Perhaps the engine builds atlases from all of the sprites at runtime, like a lot of engines do, so whether it's in one bitmap or five in the files is irrelevant
It's not terribly hard to deal with the spritesheets, though; you can just look at the ratios of the angles and set up the guide pixels very easily. Look at all of the Vanilla stuff, and the ratios are all pretty much the same. You can change the total size, but not the tween ratios or the angle. So, it has to have the shoulder and the wrist in the same positions, in terms of the angles, as every other AHuman; how you handle that graphically is another thing entirely. IIRC somebody did a tentacle-actor mod and while it was a bit gimpy it could probably be done in a polished way.
Since pretty much every actor's fairly hardcoded in terms of skeletal IK and the engine's going to pick one of the bitmaps according to the length of the distance between the arm's parent and the hand atom (which is mainly determined by the stance requirements of the HeldItem that's in the hand right now), then rotate it, you can't really do a lot of things you'd expect to do if there was an arm, lower arm and wrist.
Note that the hand just expects a bitmap, because it's going to be wherever that line segment is, period and then rotated to the vector the item needs to be at, which is partially controlled by the item's own stance offsets. So basically we don't get much control at all.
Why it was done like that was probably to keep the number of MOIDs used per AHuman as low as possible, since they're such a big performance hog.
I still wish they hadn't bothered with four MOIDs for legs though, which I really think is a waste, and had instead used those MOIDs to make arms have all three joints, though. I'd rather see full sim there, where it's most useful for flexibility's sake, and have legs use simple state machine animations, which would require more content but be a lot faster. But obviously that's not going to happen at this late date
