I recently saw that Mighty Max had put out a multiboot loader for the GBAMP (http://mightymax.org/gbamp_multiboot.html). Since this is the method I use to load code, I was very interested. Imagine my surprise when I found there was already support for 3rd party addons. Around one hour after finding that out, I had a working .mod player that runs using the multiboot loader.
Get it here!
To use this thing, unzip the .nds file to mb_data/apps, the .bmp file to mb_data/icons and the .mbe file to mb_data/ext. Now, just add .mod files to any random directory on your compact flash card, and select them using the multiboot loader.
Enjoy the first release!