I think you could just add them with a 1K resistor, only adds a few mA of load to whatever is driving the lines - like D13 did with the LED on Duemilanove's and the original Uno (on Rev 2, Rev 3 they used the LM358 op amp to drive it instead). Or use a 5K resistor for less load (~0.5mA) and just a dimmer indication.