using arm_navigation_msgs/GetMotionPlan: what setup/nodes are required to receive trajectories programatically

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You would also need to setup an environment_server (package planning_environment), for keeping track of the robot state. If you're only interested in self-collision checking, make sure that the use_collision_map parameter is set to false.

If you plan on specifying task-space goal configurations, you should also set up an inverse kinematics service/plugin.

Depending on which planners you use, the output might not be a smooth motion, nor properly parameterized in time. Trajectory filters (smoothers, shortcutters) come in handy for this. Of course, once you bundle OMPL + environment_server + IK + tajectory_filters, you have thrown most of arm_navigation in, which is what you're trying to avoid. At any rate, I hope this post helps at least to determine what functionalities won't be available in minimal setups.