Due to much demand, we are releasing a DLDI driver, with source, for the DS-Xtreme. We originally planned to also release a "pre-DLDI" libfat driver, but unfortunately that code has now become dated, and it would be trivial to derive a new libfat driver from this DLDI code.
The source should be commented well enough to follow, but if anyone has any questions, feel free to post them in our home-brew discussion forum here!