AC overvoltage protection for bike dynamo (alternator)
There are two approaches I would suggest.
Simplest approach
Permanently connect a power resistor in parallel with D1 to provide a minimum load on the alternator. e.g. 220 or 470 ohm. This will prevent the output voltage from rising too high but has the disadvantage that it will 'steal' some power from the load. Nevertheless, you may be able to find a compromise in the resistor value, i.e. a value that prevents an overvoltage yet has negligible losses.
More complex
Connect the following circuit across the load terminals (after the rectified output of the alternator). If the voltage rises above 10.1 + Vgs_threshold (the threshold voltage of the MOSFET), the Mosfet will turn on, effectively connecting the 100R resistor across the load. This will then reduce the load voltage, protecting your voltage-sensitive components.