The reverse current of a single diode is higher that you realize. The reverse current through a diode bridge is a lot lower because it has to go through two diodes in series, as well as two coils.
The only schematic I can access right now is for a Toyota alternator. It is interesting as it is an ungrounded wye wound generator, but the diode bridge is delta connected to the coils. When the engine is off, parasitic current through the alternator has to go through the three diodes on the + side, then through two coils per diode in series and then through the three diodes to the neg side. In other words, there are three parallel paths each involving a diode, two coils and another diode in series.
Now the book I am looking in has two alternators, one with an internal IC regulator and one with the mechanical external regulator. In the mechanical regulator, there is a set of contacts that open when the engine is off that takes the alternator out of the circuit, but it looks like a shorted diode on the + side might energize the coil and make the contacts.
The internals of the IC is not shown, but the alternator has a total of 12 diodes with the IC regulator. 6 are in the bridge, the rest support the IC.