I've been messing about with the demo disc a bit more.
The menu system for the disc is a webpage, looking at the index.html we can see it has code for another button at the bottom for the Pokemon distribution, this is disabled by removing the hyperlink and pointing the image to a blank png file called spc.png
index.html
<tr>
<td><img name="pokemonrs2" src="images/spc.png" width="238" height="61" border="0"></td>
<td><img src="images/spc.png" width="1" height="61" border="0"></td>
</tr>
So as the images for what would be the Pokemon distribution menu item exist on disc, we can just edit them back in, along with the hyperlink to the page which launches the distribution which still also exists on the disc:
<tr>
<td><a href="pokemonrs/pokemonrs_p.html" onMouseOut="MM_swapImgRestore();" onMouseOver="MM_swapImage('pokemonrs2','','images/pokemonrs2_f2.png',1);" ><img name="pokemonrs2" src="images/pokemonrs2.png" width="238" height="61" border="0"></a></td>
<td><img src="images/spc.png" width="1" height="61" border="0"></td>
</tr>
End result, the main menu with the hidden distribution unlocked.
A menu with the distribution option unlocked also exists on the disc as index2.html
This looks exactly the same as my edited menu above, however it does also have some additional meta tags.
For example it shows we can define keys on the GameCube controller to open a specific html file:
<!-- button settings -->
<meta name="keyY" content="index2.html">
If this had been present on the main index.html I would have speculated it could be a hidden way of unlocking the distribution, however its not. So i'm unsure why it's present in the unused index2.html, as all it does is re-load the menu you are already on.
It would be pretty easy to translate the disc menu system too if we wanted to, although pointless given all the video content and demos are Japanese.
Below is a simple image edit:
However if we could one day build custom TGC's that distribute other Pokemon, it would be easy to create a nice menu system to launch each distribution from one GameCube disc.
The main problem with that is when editing GameCube ISO's you can only replace files with other files which are the same size or smaller. That could be worked around by totally extracting the ISO then rebuilding it with GameCube Rebuilder. However i've not been able to re-build a working ISO.
This is also the reason i've not been able to modify the TGC which distributes the Zigzagoon, as the TGC has to be converted to an ISO before we can extract the contents or modify it. (Unless anyone knows of any tools that can work with TGC's directly?) The problem then is something like the Jirachi distribution ajxpk got working on an emulator is too large to inject in to our ISO, so again we need to re build the ISO which breaks it.
You can now see how easy it is to modify the demo disc, all you need is GC-Tool and your favorite text and image editor.
If nothing else you can now patch the demo disc to enable the distribution, using all assets from Nintendo so the patched disc is a legit as possible.