Brian,
Even though this doesn't directly apply to a cooperative effort, Michael Smith shared his experience with Black and White Magazine (for collectors) with me. He indicated that he advertised there a year (double page layout) and after that year a collector contacted him and bought his work. The collector indicated that he waited that long to see if Michael was "serious". I think that his experience directly speaks to the need for a commitment to a continuing effort.

My thoughts are that if one were to enage in a cooperative effort that a full page ad would be indicated at the outset and that as photographers were added the size would increase to a two page spread. A different segment of those photographers work represented by the cooperative would be featured on a rotating basis. I think that the existing cooperatives, while better then nothing, often have their impact lost among the little 1/4 page ads at the rear of the magazine

Additionally, the magazine advertisement would list a website URL in which all photographers represented by the cooperative would have their work shown and a further link to an individual website if desired.

These are my thoughts on the matter. I am interested in hearing the thoughts of others.