Does anyone have a clue if leaking transmission fluid can wreck an O2 sensor? I just had the seal on the back of my transmission replaced, but my codes say injector fauld, and O2 sensor has no signal.

If the seal leak on the transmission caused oil to the thrown onto the outside of the O2 sensor, then yes, it could cause the code to be tripped. The O2 sensor body allows outside reference air to enter it to be used as a comparison to the exhaust gas. If the outer shell of the O2 gets oil, or mud, or any other contaminent on it, it will prevent the reference air from entering. This will put the sensor out of range and cause the fault. I am not aware of any method of cleaning the sensor that wouldn't also damage it further, so you may have to replace it.
People that drive modern four wheel drive vehicle off road a lot encounter this same problem with mud splashed on the O2 sensor.

No, the tranny fluid will have no effect on your sensor. Check the wiring.