With my good friend Buzz, we have been working last past few months on a colecovision project called
31 in 1 : a home made cartridge with 31 games.
All the hardware part is well described on buzz web site, with schematcs and eagle files : here
Using the SDCC compiler and libraries and documentation of Daniel Bienvenu, i have written the loader part of this cartridge that let you choose the game you want using the pad.
All the source code is provided under BSD like license here : 31in1-rev2.zip
It contains also the eprom file i've used for this demo :
Finally this archive contains all files needed to build your own cartdridge (eagle files for the PCB etc ...)
If you are interested in making your own cartridge here is a model you can print :here