GA below CAT IIIa is something we have to train every 6 months in the simulator, be it due to blocked runway, alignment with the runway edge instead of center or any other failure. Usually an AT failure is thrown in and it is expected that we continue with manual thrust.
For CAT IIIb there is no GA below DH as there is no DH for us, just a mandatory GA whenever the RVR drops below 75m. Since we exchange the old fail passive 737s at the moment rapidly with the fail operational ones CAT IIIa and DH will be a thing of the past soon.