A spinning Black Hole would indeed work, but it would in fact be a
glorified conventional rocket. You see the BHs in abnormal galaxies
send out jets of gas at near the speed of light. A BH also sends out
gravitational waves.

I am a little bit concerned about the safety aspects of metric
engineering. If it doesn't and cannot work, which I sustect is the case
there will be no risk.If it does work there will be enormous quantities
of negative gravitational energy arount. Those are just the conditions
in which INFLATION can take place. Indeed one cosmologist has proposed
that we live in a spinning Black Hole and that Inflation took place at
the event horizon.