You can now buy perforated Zee shaped metal strips that are used as an alternative to the standard 2x4 for “above sheathing ventilation”. Put air intakes at the bottom and vent the top. Makes some sense.

http://www.metalbuildingdeveloper.com/Default.aspx?PublicationID=4&ArticleID=1321&Title=Energy+savings+technology%3a+ASV+improves+a+building%e2%80%99s+energy+efficiency