Hello everyone! I had the luck (and personally the honor) to meet the designer of Goin' Quackers, the Game Boy Color version. xD He told me that there are some hidden beta levels, including a beta boss battle, which can be unlocked with some secret codes. Unluckily he doesn't have them anymore... I'm starting this thread wondering to find somebody who liked the game and is able to unlock these beta levels by hacking the game. It'd be great!
Thank you very much jamespoo, but it appears to be the code which takes to the third level of the third world.
You've probably already seen this, but a YouTube user named MrConkerGuru posted this video a few weeks ago: http://www.youtube.com/watch?v=gP0NouftPBA I've never played the game, so I'm unsure if anything he found is related to what you're looking for--given the video's description I'm inclined to say it isn't, but this could still be a valid lead for your search. With the exception of the first password (which apparently lets you walk around the demo level, something you can't do normally) it sounds like these passwords all lead to accessible areas of the game, but are never actually given to the player. That said, it might be worth contacting this guy and asking how he found these passwords. If they're hardcoded, it's likely that he found a list of every valid password in the game's code.
I must thank you very much for your post and apologize because I've just seen it right now... Again, thanks for your help, take care!