I wrote a script using MoveLayer and a temporary Group Layer to get this to work. First create a new group layer anywhere in your TOC (it doesn't matter what it's called, it will be deleted at the end).
mxd = arcpy.mapping.MapDocument("CURRENT")
df = arcpy.mapping.ListDataFrames(mxd) # Assuming one data frame
group_lyr = [lyr for lyr in arcpy.mapping.ListLayers(mxd) if lyr.isGroupLayer] # The temp group layer should be the only one
lyr_names = sorted(lyr.name for lyr in arcpy.mapping.ListLayers(mxd) if lyr.isFeatureLayer)
for name in lyr_names:
arcpy.mapping.MoveLayer(df, group_lyr, arcpy.mapping.ListLayers(mxd, name), "BEFORE")
Gist is here.