Well, maybe it's not as simple as I thought 
I found this method being called in FM DIWPS_NOTIF_CERTIFY. Maybe you should use the FM instead of the method, although the only difference I can see is method lock( ) being called before certify_completion( ).
I tried to do it here in my system but keep getting a status error.
Sorry, it looked easy at first 
EDIT: I tried a bit more and no longer get status message, instead getting message SIG 618 (signature not defined). We are not going to define here in my system so I can't help you any further that this. But as long as you have a notification with status NOCO and not COCE and you have the Signature object CMPL_NTF defined (have a look at the long text for the message), I think this should work.
Cheers,
Custodio