Like Janet, I'm not sure how ad hoc you mean.
In Virginia we have an appointed State Advisory Board on Air Pollution (SAB) that advises the State Air Pollution Control Board and our department (DEQ). SAB members include representatives of businesses, environmental NGOs, academics, consultants, and lawyers. Each year they divide into several workgroups to examine various topics, which are then subject of a report. The reports vary in effort dedicated and quality and also in degree of DEQ staff support required for their production. See http://www.deq.virginia.gov/air/sabrpts.html
Janet's "how ad hoc" question is important. The National Academies (NAS/NAE/IOM/National Research Council [NRC]) (see http://www.nas.edu/) process is to bring together panels to address particular topics in the form of book-sized reports (see National Academy Press http://www.nap.edu/). NRC staff play significant rolls in report development so I'm not sure the split of actual research and writing there is between NRC staff and the panels.
I hope this is helpful.