To set a standin path, use the aiStandin.dso attribute:

import maya.cmds as cmds
cmds.setAttr( 'ArnoldStandInShape.dso','C:/Users/SOLIDANGLE/Documents/BlueDog.ass',  type='string' )
print cmds.getAttr( 'ArnoldStandInShape.dso' )

Here’s one way to list some of the attributes on a standin shape node, and find out what attribute to set:

import maya.cmds as cmds
ntype = cmds.nodeType( "ArnoldStandInShape" )
print cmds.attributeInfo( inherited=False, t=ntype )

for x in cmds.attributeInfo( inherited=False, t=ntype ):
    print "%s = %s" % (x, cmds.getAttr( "ArnoldStandInShape.%s" % x ))