I've got a mate in Brisbane who specialises in this for his living.
He uses a product called "Twister" pads. They're diamond-impregnated scouring pads that affix with velcro to a suitable floor buffing machine, and reportedly do the job of grout-haze removal with absolute ease.
If none of the other suggestions work you might find someone down your way who's able to take care of this for you, or possibly track them down on the net and rent a suitable buffer.