NB - this is not an autoshutoff as such, but may help with your needs.
You could daisy chain all your kegs so that when keg 1 fills up, it automatically starts filling keg 2 and so on. I have a pressurisable fermenter, and I've been doing closed loop transfers, which have worked really well for me. You can transfer carbonated beer nicely with this method too.
Make sure the fermenter is elevated compared to the kegs.
Once all kegs are sanitised and purged, connect gas port on fermenter to gas port on the last keg to be filled, then liquid-gas, liquid-gas up the line until you reach the first keg. This equalises the pressure in all the kegs with the fermenter. Disconnect the gas connection to the fermenter, and hook up the liquid out of the fermenter to the liquid in of keg 1. Nothing should happen. Slowly vent some gas from the final keg. Once the liquid has started flowing, reconnect the gas ports on the final keg and the fermenter. A siphon has been established, and you can walk away and come back later. 1 of two things will happen - all the beer will fit in the kegs and you'll have no overflow, or not all of the beer will fit in the kegs, and beer will fill the gas out hose from the final keg to the level remaining in the keg.