I tried to use the Atmel uC pullup resistor on input lines to limit the current through an LED (the TIL305 dot matrix display in my case). The declared value on the datasheet is 20 kohm minimum: that means 160 uA flowing through a red LED.
The idea does work, so if you need to source that little current through a pin, you can do it.
But 160 microAmps are too little for the old LEDs inside a TIL305 and they can be seen only in real darkness, and even then they are very dim. Because of this result I have not tested how fast the internal pullup resistor can be switched in and out.
Now I will use column drivers as outputs with proper external current limiting resistors. I will probably use small 1/8th Watt resistors to keep everything compact.