> I'm guessing there could be some kinda conflict if there are 2 items with the same hex value. And other stuff like that. ...
Yeah, something like that. Let's keep it vague.

> So I've heard the DLC.bin file has been found. Is this true?
Partially. The DLC info has been mostly figured out (see earlier on this thread for .BIN files you can upload to USB Gecko -- or get one from a friend). Don't let the ".BIN" name fool you, that's only my repackaged versions.
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Slightly different topic, but related:
The true DLC data comes from the Nintendo servers. For those who want to take a look and figure out the encryption, here's where the game gets them:
http://nwcs.wapp.wii.com/ruu/rvforestdl_jp.enc
[replace "_jp" with "_us" or "_eu"? for other versions]
Right now the _jp download contains a 9216 byte file for the "top". Earlier it was a 9248 byte file for the red pikmin hat (released items one-at-a-time depending on the true calendar dates - not game dates). The _us URL is empty right now.

So if you are looking for a tough challenge, figuring out the encryption scheme may be another way to distribute custom DLC.