As much as I loved this game, I'm not too crazy to try and disassemble it completely like I did with Castle of Illusion. But I wanted to mention the things I found here and the page that GoldS wrote up because this was a great prototype for it's day.
First let me show you the first thing I found that also happens to be my favorite discovery of the bunch:
Loading the subroutine at $1448 before the next main game loop while in a level will cause a "coming soon" screen to display (the "coming soon" graphic was found before but not the fantastic drawing in the background!):
Trying to load this up during any other part of the game will cause it to load improperly, so this was definitely shown immediately after completing a level at some point (probably at CES).
Loading the subroutine at $3C82 before the next main game loop while in a level will cause a "Work in progress" screen to display:
This will reset the game and load the next unused thing.
Loading the subroutine at $1C70 before the next main game loop while at the title screen will load up an unused demo mode (used in the final, but disabled in the proto). The most interesting thing about this mode is that it covers 1-1, 1-D, 1-M, 2-1, 3-1, 4-1. You can press start at each demo to go to the next one. When you reach 4-1, it'll go back to the Sega screen when you press start. The only demo that kind of matches with the level layout in the game is 1-1, everything else doesn't seem to match.
Loading the subroutine at $3F5E before the next game loop at any time will display this:
It says something like "Now, let's go Donald!" (thanks franz for translating :*).
There are three other subroutines that are unused in the game's game mode array but I don't know where they're called or what they do besides alter the game slightly.
1.) 155A - more use in game it seems. Camera stops moving on one plane, main player sprite is partially garbled, hud disappears. Doesn't seem to load anything into VRAM.
2.) 1CE0 - doesn't seem to do anything in game but will cause the game to run in a NOP'd loop if loaded elsewhere
3.) 3CF2 - does something similar to 155A...
I went through the entire game and did an active disassembly with Exodus as thoroughly as I could and managed to make a pretty good guess on what's used and what isn't. I think I managed to pretty much load up every used object and stage in the game, so any address 'predicted' by Exodus is probably unused objects and other stuff.
I probably wont look at this game too much since I'm working on some other projects at the moment, but I'll probably return to this sometime in the future.