Just a small update. After reading the code, connecting pin 4 to IBUS Signal was enough. I also supplied Arduino with GND and VCC from IBUS. Worked well at first, I also read the voltage out and transmitted it, by voltage divider (100k/1M, so I get safe voltages at A0). Only limitation was that I was able to transmit only integers, so I ended up transmitting 10x of the real value (so my controls shown 99V instead of 9.9, but as I need it for low battery alarm only, I can always set it to 90V instead of 9.0V)
When there was no motor on ESC it worked fine. Then I added a motor to ESC, and tried to control it. At this moment the GND cable between IBUS and Arduino went up in flames, and second later, the ESC (cheap Chinamade 320A part) also started smouldering. Fire alarm also started howling, at midnight.... After I extinguished it, stopped alarm, opened windows, and brought my wife and daughter back to sleep, I realized that the ESC probably had no common ground on Battery side and on Receiver/Servo side, so when only one idling servo was a load, it went well, and with a big can motor taking several Amps the entire current flowed over IBUS and Arduino to the battery GND (my sensor was at balancer plug). Luckily the thinnest wire acted as a fuse - the cable was a cut protoboard connector..
Arduino seems to be optically intact, same as receiver, will need to check if they are alive tonight.... Should have been only flowing over the GND on the board.... ESC is dead for sure.
So, my task is now to make a ground-isolated sensor for voltage