For occasional fun, I'll cruise around ebay and amazon and newegg and similar sites for some inspiration and to see how cheap stuff from China is. Electronics shouldn't get designed in a vacuum, eh? A few ideas for this project came together:
Cheap development boards have been around for a while, search for "TFT LCD Development Board". Right now I see some good options for 30-40$. I suspect it takes some effort with suppliers to sell for that low, even from China. The ARM boards have some neat features like super fast SD card read and the processors are cheap on DigiKey. They compile with GNU tools. I should try one in a project, to figure out how they work. There's probably lots of tutorials out there.
The bigger LCD screens are not expensive either. The 7" one I'm looking at cost 20-30$. I want to tear off the plastic to see what lies underneath. Looks like they use composite video, so I expect some electronics for a conversion to parallel port from RCA in addition to a switcher. Some people stick that directly into a Raspberry Pi (separate power), which doesn't sound like a bad deal. I like the STM32 idea better because a Pi would be a little bulky for my taste. May be a good backup plan though.
So I'm going to buy some things and figure out how they work. There's lots of datasheets to read. The overall plan is to have a boost from battery voltage to whatever the LCD needs, store some visual data in a microSD card, read it from the SDIO on an ARM M3 STM32F103 chip, maybe add additional visuals, and use some GPIO as parallel output for the LCD, with the HSYNC and VSYNC (or whatever it's called) according to a spec sheet that looks like it might be what's inside.