Composite video is named as such because it contains color data (chroma), brightness (luma) and synchronization on wire.
1. Chroma is not important in this case becuase the gameboy is monochrome. All of the color differences will be made with Luma.
2. Luma: As above, the Luma will be used to create our four different colors. The TV looks for an analog signal here. The gameboy has two data pins which are used for this purpose, so connected through a DAC of some sort, we can create four different levels rather than just 0v and 5v.
Csync or composite sync is an XOR of horizontal and vertical sync. These two signals tell the television to move down one line or to move back to the top of the screen. I am not quite sure how progressive or interlaced are determined by these just yet.
Next, these have to be connected together over one wire. As mentioned, the luma is an analog value but what I forgot to mention was that it is a positive signal whereas sync is negative. From what I have read, composite must be AC coupled because the TV input is DC coupled. That being said, our sync can actually be positive as well, as long as the luma is offset by DC value. Once the signal goes into the TV, the DC coupling circuit will block the average DC offset which is our black level. Black level will become 0 volts inside the Tv so sync will become 0v to -0.4v and Luma will be between 0v and +0.7v.
I am not at liberty to share my schematic at this point but it doesn't' work that well anyhow. :P
As a first attempt however, the result is quite promising.
You will notice that the image scrolls in several directions and that it is repeated three times over from left to right. I believe this is a problem with synchronization and possibly the LCD itself.
Updates to follow shortly.