Got it, using “Light and Color” as “Color Mode” in the “Mental Ray Baking Options”, instead of “Occlusion” which uses some other temporary ambient occlusion shader.
I had a “Error: Invalid Color Mode” at one point but restarting Maya seems to a have fixed that. Also my ambient occlusion shader is highlighted red as if hinting that it is not working, but that didn’t matter—I am guessing this is just a warning for MentalRay nodes used in MayaSoftware render layers.
Since I’m feeling kind of foolish, here is a relevant code snipet, used to automatically generate an ambient occlusion texture file using a bake preset and a mib occlusion preset:
# get post-bake lightmap project output folder file name list lFileNameList = [lFileName for lFileName in os.listdir( lOutputFolderPath ) if not lFileNameList.count( lFileName )]
lFileName = lFileNameList
lFilePath = lOutputFolderPath +lFileName