Taking the hybrid route is tougher to accomplish than it was in the series regulator. Because of the high idle current, the TL431, the obvious choice, is not useable.
(One trick might be to use the TL783 in an wildly different configuration. I have not actually built this shunt regulator, so no guarantees are offered. But the premise seems reasonable enough: the TL783 is controlling the current through the 300B. If the output voltage climbs higher than 400 volts, the TL783's adjustment pin sees a voltage closer to its output pin's voltage. As far is the TL783 is concerned, the output voltage is collapsing so it increases its conduction to compensate for the dropping output voltage. This increased conduction will increase the voltage drop across the shunt resistor Rs and bring down the voltage.)