Wow, nice reverse engineering. I think I got the main idea, more or less.
So this is a completely new compression? If there is no (de)compressor, I guess I'm out of luck.
Using SJIS I do get some results like 有力者 but looking through the memory seems to be a better solution:
The file was twice in memory: compressed and decompressed. I hope this helps.
One hypothetical thought, would it work to fetch the data from memory, change it, put FF every 8 bytes (and 05 every block?), place it back into the .pack files and adjust the stored size and the hash (however it works)? It'd be subpar, but at least I could skip compressing & decompressing.