Available in version 43 and later.

SetControllerArg2(<-partUID <-controllerNumber <-arg <-val)

Sets a controller arg for a part. ControllerNumber is the int position of the controller. For a part with only one controller, it the controllerNumber would be 0. Arg is the string name of the arg. If arg does not already exist, it is created and attached to the specified part, then assigned value val.