This script moves the selected object to be centered on the world origin but it sits on the z plane rather than half of it being below the z plane.

 

import bpy
from mathutils import Vector

# Put the origin at the center of volume
bpy.ops.object.origin_set(
type='ORIGIN_CENTER_OF_VOLUME',
center='MEDIAN',
)

for obj in bpy.context.selected_objects:
# The bounding-box corners are stored in object-local coordinates.
# Transform them to world coordinates before finding the lowest point.
lowest_z = min(
(obj.matrix_world @ Vector(corner)).z
for corner in obj.bound_box
)

world_matrix = obj.matrix_world.copy()
world_matrix.translation.x = 0.0
world_matrix.translation.y = 0.0
world_matrix.translation.z -= lowest_z
obj.matrix_world = world_matrix