I don't know if @supercomputing was doing so with API call. But here is the code I used to compete with @supercomputing, you can see, only need one line to get the public key:

//construct pow operation
          chain::pow_operation op;
          op.block_id = block_id;
          op.worker_account = miner;
          op.nonce = start + thread_num;
          op.props = _miner_prop_vote;

// some code omitted here to find a working sig from local db

               op.work.signature = sig;
               op.work.work = work;
               op.work.input = op.work_input();
               //calculate worker
               op.work.worker = fc::ecc::public_key( sig, op.work.input, false );

// construct transaction
...