After much confusion and irritation in 2007 on the policy towards group certification within the NOP - there seems to be a solution and clarification well on the way.
Group certification was conducted for NOP products all the time - with probably most products imported from outside to the US, originating from such group certification schemes. Still it was found somehow not according to the rules as not regulated within the NOP respectively.
This article in SUSTAINABLE INDUSTRIES indicates that a move is imminent. the article refers to the agenda of the next Standard Board meeting (NOSB):
"How to simplify organic certification for multiple grower sites and refining the "100 percent organic" label are among the agenda items of the November meeting of the National Organic Standards Board (NOSB).
An agenda item about certifying operations with multiple sites or units may result in a rule that allows inspectors to visit a sampling of a groups’ sites. It is based in an effort to make it easier for small producers in less developed areas around the world to be certified, according to a recommendation to pass the rule issued by Compliance, Accreditation and Certification Committee."