I need one more digital output! Can any of these be made to work this way?
No. A6 and A7 can only be used as analog inputs. You can use A0-A5 as digital output/input as well as analog input.
You can add external hardware to give you more outputs if you like or you can move up to a different microcontroller with more outputs. The ATmega1284P is a popular intermediate between the ATmega328P and the ATmega2560 used on the Mega. If you want to save a little money and don't need the extra memory of the ATmega1284P you could consider the ATmega32.
Zip_Ferndale:
I need one more digital output! Can any of these be made to work this way?
What are the pins being used for at the moment ? Some devices, for instance an LCD display, use multiple pins but are also available with an I2C interface which saves quite a few.
D2-D8, D12, D13 and A0-A5 as outputs, all to multiplex RGB LEDs, D9-D11 PWM
You can use RX but that pin is also used to upload sketches to your board and to receive serial input (from Serial Monitor, for example). Connecting something to the RX pin could cause you to no longer be able to upload to the Nano and will break serial input.