You sure about this? As far as I know, the 24h layout has been the same since the opening of the GP Track in '84, and the Müllenbach-Schleife has been used in the 24h race ever since. The reason why they don't use the Mercedes Arena (nicknamed "Haug Haken" (Haug Hook), named after Mercedes Motorsport Director Norbert Haug) is, that it just did not exist until 2002. Adding it to the 24h race would've changed the lap times quite a bit.Because for the real 24h race there are several layouts possible. Sometimes they don't even go until Dunlop-Schleife and take the DTM shortcut (like in the DTM event in GT5). Could be possible that leaving out the Mercedes arena had been done during the race that Kaz participated.
The short track without Müllenbach, but including the Haug Haken, is used in the endurance races of the VLN series.
There are some great sites out there with tons of infos on the Nordschleife, like Mike Frison's 20832.com and mikefrison.com, bridgetogantry.com and nordschleifologie.de. The latter is german only, but has all the history, infos about the Südschleife and a lot of great pictures from the past.