I've had a play but can't get anything to compile - very strange.
If you don't use the LED in the "Caps Lock" switch all you need to do to use that pin is change the 2's to 3's on these lines in xd60.h
inline void xd60_caps_led_on(void) { DDRB |= (1<<2); PORTB &= ~(1<<2); }
inline void xd60_caps_led_off(void) { DDRB &= ~(1<<2); PORTB &= ~(1<<2); }
Then in rev2/config.h change the B3 at the end of the column pins to B2 and the B2 for CAPS_LOCK to B3 on these lines.
#define MATRIX_COL_PINS { F0, F1, E6, C7, C6, B6, D4, B1, B7, B5, B4, D7, D6, B3 }
#define CAPS_LOCK_LED_PIN B2
and compile it.
Using the backlight pin is not so easy as it's attached to a dimmer circuit - that was a first thing in the morning not awake suggestion! Using it would require working out where it goes and removing the component so that's the last option to try.