I went back into PP2010 to further play with this concept and understand exactly what curtails when custom_output_3 is enable for PSD export.  As the manual describes, custom_output_3, when no materials are associated with it, will create a PSD layer that contains Shadow only.  This will also create a transparent background, similar to saving in PNG format.  The OP's problems is strictly capturing ground shadows into a separate PSD layer for postwork, which is plausible but will involve more manipulation to get the final results.  I think we are all in agreement that the original scene is too saturated in IBL light sets to effectively cast any significant shadow.  And this shadow effect is strictly dependent upon AO presence.  One IBL should be enough for cast marginal shadowing, but it's intensity may not be relevant, depending on the postwork scene.

As a test, I loaded Andrea into a vacant scene with one white IBL, 15% intensity, placed overhead and quickly rendered the scene with Custom_output_3 selected in Render settings.  The resulting image was then exported as a PSD and opened in PSCS3.  The enclosed image shows the results.  Please ignore the file title, that is incorrectly labeled.