I know you are not interested in U.S prisons, but I can’t resist.
Robert Stroud was allowed to write about birds as USP Leavenworth because prison staff thought it would keep him out of their hair. (Stroud was both habitually violent and a major pain in the ass.)
At USP Alcatraz, the warden made the decision to let Stroud write about the prison system for pretty much the same reason. Stroud was hated at Alcatraz by guards and inmates alike and keeping him busy reduced the problems he caused. His prison manuscript was never published but writer and Alcatraz historian Jolene Babyak did obtain a copy. She told me that it wasn’t published because it’s mainly Stroud’s rantings, it offers no original or significant information, and it’s highly redundant.