So I basically resolved this issue myself by working within FlxSprite and not trying to dick around with BitmapData. It also dawned on my that one of the objects I was trying to get the BitmapData of was an FlxSpriteGroup which is why it was coming up null but not throwing a type error. Anyways, this is my FlxGroup flattening solution in case anyone wants it.

public static function flattenSpriteGroup(source:FlxSpriteGroup):FlxSprite {
	/*
	 * Flattening function, useful for when you're trying to construct 
	 * an asset from multiple different assests, such as a multi-layered tile
	 * or a character sprite.
	 * 
	 * Only works for sprites of the same size at (0,0). In theory should have worked
	 * for any sprite sizes, but there's some weirdness with sprite positioning that
	 * I can't be bothered with figuring out. 
	 */
	var finalSprite:FlxSprite = new FlxSprite();
	source.forEach(function(sprite:Dynamic){
		var workingSprite:FlxSprite = sprite;
		if (sprite.graphic == null){
			//Check if sprite is a group
			if (sprite.group != null && sprite.group.length > 0){
				Log.trace("recursion");
				workingSprite = flattenSpriteGroup(sprite);
			}
			else {
				//If no group is found then sprite just has no graphic so we ignore it
				return;
			}
		}
		if (finalSprite.graphic == null){
			finalSprite.loadGraphic(workingSprite.graphic);
		}
		if (workingSprite != null){
			//finalSprite.stamp(workingSprite, cast(workingSprite.x, Int), cast(workingSprite.y, Int));
			finalSprite.stamp(workingSprite, 0, 0);
		}
	});
	return finalSprite;
}

So my end goal is to take a sprite group and flatten it into a single sprite. Now to go about this my plan has become:

1. Iterate over sprite group.
2. Run updateFramePixels() to get the BitmapData
3. Merge transparent BitmapData onto a common object
4. Create an FlxGraphic from the BitmapData.

Now the weirdness comes in that the original sprite group is not currently being rendered on the screen. We're in a substate, and the original sprite group is in a state that's not being drawn or updated. I suspect that because of that even forcing a drawFrame isn't suffecient to get updateFramePixels to return bitmap data.

Now I would like to make this a generic static method, so adding the sprite group to the current scene and then deleting it seems a bit distasteful. So I was wondering if there's either A) A way I can get updateFramePixels() to return the BitmapData. B) A different way I can pull an FlxSprite's BitmapData. C) A different approach I can take to "flattening" an FlxSpriteGroup?