As you're already aware, there's no way of adjusting the alternator output, as you can with a car alternator (by varying the field current). So all you're doing is trying to stop the battery over-charging.
Back in the 60's Triumph did this by using a LARGE stud mounted zener diode, directly across the battery - mounted on a heatsink, and positioned between the fork legs, to get forced air cooling.
Such zener diodes would be expensive, so it's easier to use an amplified one, this is a simple example, the transistor needs to be a power type, able to sink the output of the alternator, and needs to be mounted on a decent heatsink where it get's good air flow.
As an alternative you could get far more complicated, using opamps or comparators feeding a power transistor - but something like this might be enough for your purposes?.