Builing a frequency inverter chimera
Yes. Fully regenerative variable frequency drives (VFDs) have been available practically throughout the history of VFDs. The characteristics of the AC source are not exactly the same as the characteristics of motors, but the motoring and regenerating functions can still be incorporated into the same control system. The cost is too high to justify using regenerative VFDs where the only regenerative function is occasional braking. They are most attractive for controlling dynamometers used to absorb power when testing engines. They have also been used in processes in which one motor pulls and stretches a material while another motor holds the material back. An electric vehicle like the Tesla that uses an AC motor could also benefit, but I don't know if they are doing so at present.


  Question is, if the rectifier stage of one inverter can be removed, and replaced by the same circuit as the output driver stage taken from another inverter. Like we would treat mains power just as another motor.


Yes. Something like that can be done. The same switching devices connected in the same way are used in the input and output power sections. The control section needs to have appropriate software programming and the capability to provide the necessary signals to control the switching devices and the feedback sensing and signal system needs to be configured appropriately. Mains power can not exactly be treated as just another motor. In addition to the feedback signal and software programming mentioned, some additional filtering is required.

Emerson / Control Techniques apparently has Unidrive models are configured so that two identical units can be connected as described. The system is described in some detail in "Control Techniques Drives and Controls Handbook" (see diagram below). Other manufacturers may use the same or similar input and output bridges and add other components as required in a product that is specifically constructed as a four-quadrant, regenerative drive.