The
Pokémon mini is a Nintendo handheld released in 2001 which, to the external viewer, was basically like a tiny, monochrome (not greyscale) Game Boy. It took small cartridges and about ten commercial games were released for it. It was later reverse engineered and has had a number of homebrew applications developed for it and at least one GPLed (Windows) emulator:
Minimon.
On the upside are the devices generally low specs and the display size (96*64), as it's small enough for almost all supported devices to display in full; e.g. Sansa's Clip series are 128*64, almost a perfect match.
On the downside, the device is almost entirely custom; barely any standard parts, so its not as well understood as other architectures like the Z80, etc. Still, there's a
fairly comprehensive wiki on the hardware, as well as the aforementioned emulator for a practical approach.
Brief spec rundown:CPU: 8-bit custom 4MHz, nicknamed "Minx" by hackers
RAM: 4KB SRAM
Input: D-Pad, two face buttons, power button, "shake" (pedometer)
Audio: Custom single channel square wave
Display: 96*64 monochrome LCD
The device also features some less important emulationwise features like an infra-red port and a vibration motor.
All that said it might take some optimisation to get any sort of decent speed out of the emulator on some of the lower-spec players Rockbox supports, and less importantly, the device's primitive "greyscale" mode may cause issues. Really it's just blinking pixels at a faster refresh rate than the display can handle, "blurring" into additional shades--for those familiar with homebrewing TI's graphing calculators, it's the same "trick" used there--but since Rockbox is seen on a vast array of displays, it might be complicated to get this working on all screens; still, the games rarely or never
rely on it, so while it might look a bit ugly, not supporting it would cause no major issues.
Besides its lack of attention previously in terms of emulation on other platforms, it seems like an ideal target for emulating on DAPs.