I imagine that the GBA screen would not be similar to anything else, and would have a proprietary controller or similar, simply to make it harder to reverse engineer and annoy those who are inclined to attempt modding it.
That said... placing it inside a GBA SP, would be excellent. You could even mould a new face plate with holes and a bit of extra room for a keyboard.
EDIT: A quick google tells me that very few people have tried any hacks using the GBA screen, and the fact that it is fixed-res with a custom interface makes it less than preferrable to use for a PC. However, you should just be able to use another ~3in LCD for the purpose.