I have personally tested modding this app with custom games.
The only buttons that do not work are L2, R2, and Start. The Plus button specifically is mapped to instead press Select, as is the Minus button. The lack of Start will hurt many games, like Chrono Cross for example. With that you cant get past the title screen that requires you to press start.
You specifically need to point to the cue file of a bin/cue pair.
While the carbon engine allows for some customization things in a xml file... a little too much is hard coded. Particularly the main menu. The hard-coding overrides any changes to the related options in the xml. So at best, you can probably replace many of the menu screens .pak files with a .pak containing a single custom image to somewhat customize the UI. Just not enough to my taste. Perhaps to others though.
As I suspected, and Alan proved with testing... the .stt files are save states. The ones zeroed out per Alans instructions above work like SP title states from NSO, in that they are just loaded immediately on start. This is to bypass the intro of the game.
The apps intros can be skipped by modding in this to the gamedef.xml
Code:
<intro>
<Splash1 assetName="" company="" />
<Splash2 assetName="" company="" />
<movie id="0" assetName="" endFrame="" />
<gameIntro assetName="" />
<gameOutro assetName="" />
<gameCredits assetName="" />
</intro>
The above skips all but the Carbon intro. That can be skipped by making a 0kb LRGCarbon.mp4 file in the assets/splash folder.
Overall... this app could maybe be hacked to remove the hardcoding, and possibly remap the plus button to Start... But its not something I will be investigating myself right now as I have to focus on getting CaVE back to a public release state. In the future, apps using this engine might be supported with CaVE similar to how ZebraEngine is for Saturn... But its a low priority right now over getting CaVE v1.5 out of beta and public. More so with the obstacles of incomplete button mapping and limited UI customization in the way.
[your contract] "NON-RENEWABLE" 
