[MoveIt!] Plan to a goal without specifying all axes

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The OMPL's RRTConnectkConfigDefault planner should handle this, you just need to setup the constraint at the goal pose such that the orientation about the desired tool axis is underconstrained, below is pseudo code of how you'd set this up:

  // Create the Motion Plan request object
  moveit_msgs::MotionPlanRequest req;
  req.motion_plan_request.group_name = group_name;
  req.motion_plan_request.workspace_parameters.header.stamp = ros::Time::now();
  req.motion_plan_request.start_state = start_state;

  // Set the tool pose
  geometry_msgs::PoseStamped stamped;
  stamped.pose = pose;
  stamped.header.stamp = ros::Time::now();
  stamped.header.frame_id = req.motion_plan_request.workspace_parameters.header.frame_id;

  // defining constraints such that tool z orientation constraint has a large tolerance of 1.0 
  std::vector<double> pos_constraint = {1e-3,1e-3,1e-3};
  std::vector<double> orient_constraint =  {0.01,0.01,1.0}; 
  moveit_msgs::Constraints c = kinematic_constraints::constructGoalConstraints(
    tool_link_name, stamped,pos_constraint,orient_constraint);

 // Add the constraint to the motion planning service request 
 req.motion_plan_request.goal_constraints.push_back(c);

// call the motion planning service
 motion_planning_client.call(req, res)