Not sure about screen brightness. I've been looking at GBA hacking sites, and it certainly has potential. There seems to be a plethora of software tools available for creating GBA apps, and it is supported by gcc. It is very limited on RAM though, so a full blown linux kernel is probably out of the question

The 'link' port can be configured to be like a standard RS232 UART, with a 115k maximum baud rate. But it may be more useful to use it in 'Normal SIO mode' which can be either 256kbits/s across a cable or 2.5Mbits/s if the device is local. My thinking is that a device could do an ethernet<->SIO conversion, which would provide enough bandwidth for a remote display.

With 38400 pixels, mimicing the empeg's 2 bit color would allow for ~4 updates/second on the slower rate, or ~32F/s on the higher rate. Obviously, if higher color palettes were desired then those rates would drop. But for GPSapp use, 8bit color at 1F/s (lower rate) would be fine - most receivers don't report position more than 1/s anyway.

It may be that we can get even faster access via the cartridge port and some form of DMA. I haven't found much about this yet.