... but your "original" thought is correct I think (as far as I understand), front a rear lights are definitely in parallel, in parallel downstram the overvoltage protection that is built-in and becomes "active" only once you switch-on the front light.
That's interesting. Are you saying that there is a protective device or circuit clipping the AC prior to rectification etc.? The rear light circuitry must then rectify and regulate the dc to drive its LED and standlight capacitor. - This must be the case as you can connect a rear light straight to the dynamo and it will work (for a short time!).
It would be interesting to see examples of the circuits used commercially. As mentioned in my earlier post, the only one I have seen is the reverse engineered Supernova triple, which supplies regulated dc to the rear light from within the front. A benefit of this design is that it only requires one standlight circuit for both lights.