As to your problem: Gamecube Hacking is a pretty unusual topic around here, so it might be the case that not many people around here are able to help you. If you could provide the actual file (or some other examples) you are trying to reverse engineer, we might be able to have a look at it, and just figure out what sort of algorithm it is. Since it is a Nintendo Devkits we are talking about, they were using some forms of LZ compression in the past, and chances are that they also used them on the gamecube. The GBA for example uses mostly LZ77 for compression, the Nintendo DS uses mostly LZW (as I recall) and so on. Chances are the gamecube might use something very similar.
Is this a file common among other game cube images as well, or is it specific to Harvest Moon? If it is a common file, it is possible that someone already figured out what it is, and how it works. If it's not, it's an oddity, but figuring it out should still be feasable
Also: Sidenote: If you did not notice, we are a german community, most of our members will probably speak german, so don't expect everyone to understand you