Let’s consider what canon law establishes. “Apart from the supreme authority of the Church, only those can grant indulgences to whom this power is either acknowledged in the law, or given by the Roman pontiff.
“No authority below the Roman Pontiff can give to others the faculty of granting indulgences, unless this authority has been expressly given to the person by the Apostolic See” (Canon 995).
Consequently, unless a priest has a specific permission to place a plenary indulgence on a rosary, he can not do it.
The Enchiridion does provide a daily plenary indulgence to anyone who recites the Rosary in common – that is, with two or more people reciting. But the indulgence is not for the moment of death, nor is it attached to the physical object of the rosary.
A similar question was answered in this column some time ago, and it might be useful to recall something I wrote then:
“Regarding a plenary indulgence at the moment of death, the Enchiridion states that a priest should not omit the Apostolic Pardon with the attached plenary indulgence when he attends to a dying Catholic (see No. 12.1). But if a dying Catholic cannot be assisted by a priest, the Enchiridion goes on to state: ‘Holy Mother Church will grant a plenary indulgence at the moment of death to any of the faithful properly disposed who, not assisted by a priest at that moment, have habitually prayed during their lifetime; in such a case the Church will supply the three conditions normally required for a plenary indulgence,’ namely, confession, Communion and prayer for the pope.”
Fr Rocky Hoffman