I have yet to bother to play with the firmwares gone ROM but I assume it is just the firmware with the appropriate copy/entry point and header added. Short version is yes you can edit it but it will take an awful lot of effort.
The DS firmware is a complex mish mash of compressions, encodings and oddities once you get past the basic settings. There is a program to decompress it which you can read about on
http://forum.gbadev.org/viewtopic.php?t=12270 (download it too if you want). It should include source code which is nice as there is no way to reassemble it afterwards that I know of but if you are truly set on it then that would be where I start to look for things.