The standard bootloader only uses 3 wires: Tx, Rx, Gnd. So when you say 1 wire you mean 2 wires (as you acknowledge).
I'm not sure you are saving a heap by going down this path.
In a commercial product reliability would count for something, and having 1-wire (plus Gnd) means you don't get any acknowledgement so you don't know for sure if you changed the program.