Yeah, still won't be easy to integrate the batch optimizations now, since all the code in this module is written with the single-line-at-a-time assumption.
It seems to me that this would be an area where something like reactive streams excel at. The interface is by default streaming and the code dealing with the database could decide to buffer the incoming stream to an optimal amount for creating a query. (Of course only buffering until a maximum latency was reached.) This way neither the producer nor the consumer need to be aware of the exact buffer requirements and the whole can be optimised using back pressure.