If you need only one way, this should work fine for a Serial connection (it's asynchronous anyway). You don't need both lines connected for Serial.
One thing to consider: the idle state of Serial is high, so your opto is on all the time. That's not good for battery life. You better add a transistor to switch the optocoupler and reverse the signals, so the opto is active when the line goes low. Then also reverse the output side and you transfer your signal just like that.
You may also consider to use an ATtiny25/45/85 for the voltage measuring job. Much smaller, and even the ATtiny25 should have enough memory to do this job just fine.
