I took a good look around the web, too (tour provided courtesy of Google), and couldn't find any DIY green roof instructions that didn't involve synthetic materials.
If anyone here has access to an academic database (I remember Jstor, ProQuest, Art Index, and WorldCat from my days as a student in architectural history), perhaps they may lead to some architectural studies of traditional green roofs in Scandinavia. Graduate papers on architecture can be very detailed. I know from personal experience, I had to write them. Most scholarly articles are only accessible through paid databases (or in a University library's journal collection), but those articles often cite published books that are informational treasure troves, and those books might be available at the public library or through interlibrary loan.