If I remember correctly, the "Ride of the Ringwraiths" campaign in White Dwarf had rules for Ringwraith recovery between scenarios. I don't have the magazines here at work, so I may be wrong...
I had been thinking of playing the Ride of the Ringwraiths linked to the FotR journeybook. The Ringwraiths would ride through Gondor, Rohan and Sarn Ford and start appearing then in the FotR scenarios until the Ford of Bruinen. When neither side is guaranteed to recover fully, both sides would be so bruised and battered at Bruinen that they could hardly stand.
Evil not recovering fully in the FotR scenarios might pose a problem to the play balance if the scenarios are designed and playtested to work best with full-powered Ringwraiths.
-- Pasi