All Riders:
Select "Freestyle" at the main menu. Select Sierra at the character selection screen. Select "Time Trial" at the race configuration screen. Set the number of laps to "6". Turn off all Blades, then put Edge as Blade 2. Start a race, pause the game, then quit the race. Press Triangle at the race configuration screen. Press Triangle at the rider selection screen. Quickly press Up, Down, L1, R2, R1, L2, Circle, Select at the title screen. If you entered the code correctly, a box with "All Riders Enabled" will appear near the lower right corner of the screen.

Spy Cam Mode:
Enter this code at the title screen: Triangle, Square, Circle, Square, Triangle, Circle.

Thanks to Revolution readers themos69 and Jera!