I have attached to this post a proof of concept file created using my "cartridge filesystem" tools.
It's a "direct to cartridge" port of Solomon's Key. (I didn't even bother to set the colours/mode for the title screen. Now I just have it working, I'll add the colours from the solomon's key + hack. Can anyone provide + colours for the title screen?
This is an example that should work on gx4000 etc.
This solution involves a little more work than No$Cash solution, but it opens up the full cartridge space, and works on gx4000.
The data is stored on the cart and copied into RAM. It's easier to do it this way. It's "loaded" from the cart - this is why I call it a filesystem. At the start of the cart data you will see the directory at offset &b.
This represents another way to have games on cart.
In the directory:
First byte is cart page (&80 etc)
next 2 bytes are address to read from (pages cart pages into upper rom area) so address is always from &c000 onwards.
next 2 bytes are the length.
and that is it.
So you can see there are 4 files in there.
1 small "boot" program, copied to &4000 and run from there,
1 screen, 1 main file, and another screen.
The "boot" program sets up the hardware state, loads the first to the screen, next to &100, next to the screen and executes the game.
An improved boot program will fade the title screen in and out (with correct colours), load up the main exe with plus colours.
I will post the tools and sources soon so others can use it to make plus games.
EDIT: Added the Plus colours from the Plus hack.