From the Changelog of firmware 2.4A/B
[main] Adds dual-layer support list file loading from external device
This change adds the possibility to add title IDs of games on DualLayer-DVDs (DVD9) manually by editing a text file called duallayer.list. The format is one ID per line. Usually new DVD9 games are added to the firmware's internal list. Chances are that when using the latest firmware you will never have to use/edit that list. Also note that there are exceptionally few games that ship on DVD9 for the Wii.
To get parsed on boot the file itself has to be stored inside SD:/iso/ or USB:/iso/.