How does one write debug code to get a list of energized I/O at a certain point or state of a program?

Are you talking about RSLogix 500? As I understand it, debug code will download to a emulator, but not an SLC. This makes the emulator very powerful, because you can simulate your I/O states with the debug code and watch it all run. I have used it with Emulator 500, and it works quite well.