With help from the Jackson community:

When calling the ObjectMapper:

return objectMapper
            .writer()
            .withAttribute(MaskingSerializer.JSON_MASK_ENABLED_ATTRIBUTE, Boolean.TRUE)
            .writeValueAsString(entity);

and in the serializer:

if (serializerProvider.getAttribute(JSON_MASK_ENABLED_ATTRIBUTE) == Boolean.TRUE) {
        jsonGenerator.writeString(RegExUtils.replaceAll(value, ".", "*"));
} else {
        jsonGenerator.writeString(value);
}