Pull away voltage (energy) from a DC motor driver board when it does regenerative braking
Normally Vbus stays at 48-49 volts - only when braking it raises: if
it stays under 58-60 volts there is no problem; if it raises more, the
board gets broken.

The obvious solution is a clamp that conducts if the voltage tries to go above 50 V. Something like this:-



simulate this circuit &ndash; Schematic created using CircuitLab

why add a resistor, when I have a 350VA transformer which can
dissipate energy much better? If only I could short the rectifier
bridge when needed, I could discharge the extra voltage into the
transformer.

You can't do that because the voltage at that point is a mains frequency sine wave going from ~0 V to +50 V. To feed power back into the transformer you would have to convert the DC voltage into AC with the correct voltage and phase.