object_smooth.py

import bpy

# generate the set of flat-shaded frames
def list_to_frames(ranges):
    k = set()
    for begin, finish in ranges:
        k = k | {i for i in range(begin, finish+1)}
    return k

# assumes you are OK with setting all faces to smooth or flat
def set_shading(obj, smooth_mode):
    if obj.data.polygons[0].use_smooth == smooth_mode:
        return
    for poly in obj.data.polygons:
        poly.use_smooth = smooth_mode

def my_handler(scene):
    frame = scene.frame_current
    obj = bpy.data.objects[obj_name]

    smooth_mode = True if (not frame in frames) else False
    set_shading(obj, smooth_mode)

# give frame ranges, and name of mesh based Object.
frames_to_flatshade = [[20,30],[45,54]]
obj_name = 'Cube'

frames = list_to_frames(frames_to_flatshade)

bpy.app.handlers.frame_change_pre.append(my_handler)