After many, many hours of searching, I think I've found a single chip solution (almost) for driving the LCD from the composite output of the raspberry pi. The salient features are:

• About $10 in single quantities
• NTSC, PAL, SECAM, etc. input
• "TFT panel support" with "8 bits per pixel" (I really hope this means per color)
• Supports up to 800x600 LCD panels
• Configured over I2C
• In stock at Mouser

The reason it's almost a single chip solution is that the configuration can be done the microcontroller that handles the keyboard input, but a microcontroller would need to be added to the design if the display system was used in a separate design.

And before I forget, it's the TW883* series of TFT display controllers. I'm still waiting on the datasheet from intersil (grr...) to make sure it will actually work as I inferred