Cool.
The potential issue for D13 is that D13 is used to control the LED.
The bootloader blinks the led when the board is reset.
So as long as you are ok sending 3 pulses on the D13 signal when reset is pressed, it can be used.
Alternatively, you could re-build the bootloader code to disable the LED blinking.
You would need to re-burn it using an ISP programmer.
But also have a look at the way the transistor is wired to the load.
--- bill