The idea was that Diriel would be writing her name on the things she made and that someone would find one of these and show it to Curufin so he would know his wife is alive. I think we also talked about the possibility of Diriel being caught doing this.
There wasn't any more discussion than that, so I am expanding on the idea by suggesting Diriel pass on word of a coming attack in Season 5.
If we wanted to do the armor thing, the armor, perhaps the message could be discovered by Caranthir after he helps Haleth fight off the Orcs attacking her people. Or, since secret messages in Orc armor is not an expedient or reliable way to send detailed messages, we could have Diriel communicate some other way.