This is tough, but I'm thinking of an indirect approach:
Increase the percentage of French Huguenots migrating to the Cape.
You'd need a whole brand new set of persecution for that.
- Migrating was expansive, and most of migrants preferred to go closer (basically, if you lived near a border. If not, though luck)
- Migrating was feared, and actually a threat important enough to provoke conversion
- Migrating was forbidden and harshly punished
Even if half of them, 100 000 roughly, went to South Africa (let's say that Netherlands decided to deport all of refugees on its soil there for some reason), there was a policy of cultural and political integration that would make them basically undistinguishable from other colons.
Furthermore, if persecutions are maintained for some reason (they basically went less and less a thing right after Louis XIV's death), it could help as well, but not being decisive enough.