The whole charm of X10 was it travelled over electric wires - the role of RF was primarily listening to remotes and sensors and such. Unless you got some inductivw noise issues going on the line those signals would go all the way to the poles sometimes to neighbors houses.
If you put a normal x-10 rf receiver anywhere and send it signal using CM19/12 it will take the signal and put it on the electric wire. receiver doesn’t have to be near the device it’s conteolling. There would be 3 limitations you need to consider and mitigate:
- House code you are using - x-10 receivers can only use one house code
- Phase - make sure both sender the switch you are trying to control and rf reciever are on the same phase
- Electrical noise and isolation issues - depending on the age and wiring of your house this may or may not be an issue.
Here is what i would do - first figure out what what phase the outlet where you will put the garage door is on by turning off one of the phases breakers. Then find another outlet on the same phase somewhere between where you are transmitting the signal and garage door - plug in the reciever there and test using a good old fashioned x10 remote