MoveIt visualization of planned trajectories with attached objects

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That explanation does not make sense. The PlanningScene you are talking about is part of move_group, so I am confident it keeps track of the attached objects. The bug you hit seems to be that attached objects are not part of the displayed traj if they result from a request without a start state

I understand that the planning scene is part of move_group and tried really hard to get the behavior you allude to but states returned through the movegroup API have no attached bodies...

... this doesn't work

moveit::core::robotStateToRobotStateMsg(*movegroup_->getCurrentState(), display_trajectory.trajectory_start)

this does...

attached_objects = planning_scene_interface_.getAttachedObjects();
for (auto it = attached_objects.begin(); it != attached_objects.end(); ++it)
  display_trajectory.trajectory_start.attached_collision_objects.push_back(it->second);

I also found a thread in which someone has the same issue here. In particular see the 7th email and Ioan's respective response saying that "the current state as monitored by the MoveGroupInterface does not monitor attached objects"

I see. It makes some sense that MoveGroupInterface::getCurrentState does not know about attached objects. It only knows about joint states. Could you please create an issue for this in the project so we can document or change this behavior in the future?

Your original question did not concern getCurrentState though. You wrote you submitted a planning request with an empty (i.e. no) start state and MoveIt failed to visualize the attached object in the trajectory. This is a bug in the move_group node and not a shortcoming of the interface.