I don't have any exact specifications... I poked around in it for an hour or two a while back, and it looked like a standard JJ2 file, a bunch of zlibbed blocks, except that for the most part their compressed and uncompressed length longs were themselves inside of a block which also contained their names. I could be wrong, though... I don't think I ever got around to extracting anything but that initial block. One way or other, it's all stuff you should be able to figure out by cross-applying the principles used by JJ2's other file formats.