Yes, allowing this to happen would fall under a bishop’s power of legislation, but only when done in a certain sense. The blessings must be understood in a specific way to be licit. The blessing of an extraordinary minister is completely different from that of a priest. There is no priestly blessing invoked. If a priest were to bless you, it would be: “I bless you…” usually with the sign of the cross. A layman, while he or she can still trace the sign of the cross on your forehead, cannot directly invoke the blessing in the name of the Father, Son, and Holy Spirit. A extraordinary minister’s blessing would “May the Lord bless you…” It is with the same power of the universal priesthood that the extraordinary minister bless you that you yourself use when you say grace before meals: “Bless us, O Lord,…”
So, in essence, both sides of the argument could be considered correct. If you view the blessing of the extraordinary minister as a priestly blessing then no, the bishop does not have the power to allow them to invoke that power of the ordained priesthood. If you view the blessing as simply the extraordinary minister praying to God that He might bless you, then it is a valid exercise of the legislative power of the bishop to allow it.