fs_basegame = Cvar_Get ("fs_basegame", "id1", CVAR_ROM, NULL, "game to use by default");to "default" like this:
fs_basegame = Cvar_Get ("fs_basegame", "default", CVAR_ROM, NULL, "game to use by default");
(you'll have to make a basegame directory called "default", or whatever you call your default game directory.)