Mackinac Center’s IMPACT covers Citizens United and profiles Richard McLellan. According to the article, McLellan argued Austin v. Michigan Chamber of Commerce. The article appears on page 9 of this 12 page PDF.
It is a testimony to the esteem in which McLellan was held that he was entrusted with arguing Austin before the U.S. Supreme Court despite not having argued so much as a traffic ticket at that point in his career.