I have an idea but I don't know if it would work:
Assume I have a conventional 120-0-120V system but only the two live wires are present. To get a neutral I put a center tapped autotransformer between the wires.
So far nobody has been able to tell me that this would work but nor has anybody been able to give a reason why it wouldn't work.
Now, if I have understood this correctly, the autotransformer has an impedance which means that if I put a load between either leg and the neutral the voltage will decrease. At the same time, the voltage will increase between the other leg and the neutral.
To compensate for this, I figured one might simply add taps to the autotransformer and some electronics to control them. This sounds an awful lot like a line conditioner. (I do not intend to regulate the voltage between the hot wires, only to keep the hot to neutral voltages roughly equal.)
Would it be possible to build such a device?
I'm well aware that adding a neutral wire would normally be much easier and more economical. However, the lack of a neutral could have certain advantages, like making the electricity less attractive to steal. (Code and legal aspects need not be considered.)