Hi, thanks for your interesting question.
This is not something we have tested ourselves. I have read of people using bokashi leachate in their fish tanks and ponds to control nitrate and phosphate levels. Lower nitrate and phosphate levels should, in turn, reduce the algae levels in the tank and/or pond.
Adding bokashi tea to your water butt won’t do any harm so its definitely worth experimenting. We would be really interested to hear your findings if you do trial this.