For starters we should point out that (Past)Picard didn't use the iron filing to create the message, he used a small nail that he found on the floor of the cavern. That means that his intention in leaving the iron filing inside Data's head, in a place that prevented him from being booted up must have been to prevent Data from being activated until the appropriate time.
So what would have happened if the iron filing wasn't there?
We know from an earlier episode in the season (TNG: Disaster) that Data's head can function perfectly well without his body. Had (Past)Picard not blocked Data's bootup sequence, La Forge could have brought him back to consciousness shortly before the ship arrived at Devidia 2 and before Picard and crew had beamed down to the surface.
Faced with the knowledge of how to destroy the alien's portal and the inherent risk of trying to fight against an opponent who can time-travel, (Future)Picard would have ordered the immediate destruction of the portal, regardless of the damage it would have done to the timeline. (Past)Picard would have been aware of that and is trying to preserve the timeline as best as he can by not giving (Future)Picard that knowledge until the time is right..
Knowing that La Forge hadn't conducted a diagnostic of Data's head before he left, (Past)Picard is gambling that his message to the future won't be heard until after his future self has gone through the portal but before Riker accidentally blows up the Earth.