On the Amiga 500, you can cut the Reset line and the keyboard will still work but the reset will no longer work. On the A1000, you can write your own kickstart which gets preserved even after a reset. There was some utility for a VD0: device that seems to get preserved even after CTRL-A-A.
If you are using some modified power supply that's ATX compatible, you should be able to control the on/off with a simple switch that can be triggered off from a parallel port or some other port. You can use a hard drive interface and put a flash drive or RAM drive in and have as many megabytes of RAM as the largest partition allowed by your OS.
