def set_opencl(parent_node, parm_value):
    for node in parent_node.allSubChildren():
        for p in node.parms():
            if p.name()=='opencl':
                try:
                    p.set(parm_value)
                except hou.PermissionError: #this handles the case that the parm is inside of a locked .otl
                    pass
hou.session.set_opencl(hou.node('/obj/sim'),1);