No power regulation on the transmitter module.
I was hoping someone familiar with the 433MHz ASK/OOK timing of the highs/lows could chime in, but I found a vid of someone reading the signal through a sound card and it seems shorts are ~400microseconds and longs about 3 times that. The rise/fall time of optocoupler is within the margins of recognition.
Stacked 4 coin cells for 12V, transmitter is working through the optocoupler.
Going to keep using it to see if it drops any transmissions, but it's been 100% so far.
Once it stops raining I will run it out to see if any distance is gained over the 200m range @ 5V.
So my wiring, for anyone interested
Arduino side of optocoupler:
Data out pin to optocoupler anode, optocoupler cathode to Arduino GND through a 330ohm resistor (opto's internal led is 1.2Vf, giving it ~13mA)
Transmitter side of optocoupler:
VCC and GND to battery + and -, respectively. VCC to opto collector, opto emitter to DATA pin.