Just a quick photo of my raspberry pi inside the X500 PLUS:
Keyboard works fine, there is so much space left inside that you could easily have 4-5 raspberries with a self powered hub with HDD and SSD etc...
You get the idea. 
Cool. Those Raspberry Pi boards are so small, I think you could put one in a real A500 case. Without removing the A500 first. There's a dual-boot solution!
