3-phase regulated "ideal" rectifier
Your proposed design is overly simplistic. Remember, the gate drive for each SCR is relative to its own cathode, so your common drive circuit won't work. It works in the shunt circuit because all of the cathodes are connected to a common point (ground).

Furthermore, when you put the SCRs in series with the alternator, you now want them to fire when the voltage is too low, rather than when it is too high.