Please review RVSM requirements.
Does not say that autopilot must be engaged. Also once the descent commences it's gonna take one out of RVSM airspace.
Nearing your destination at cruise altitude, autopilot and autothrottles off and maintain a smooth idle descent profile without Vnav or FD guidance, unnoticeable to the passengers and only adding power to vref N1/EPR at the outer marker with flaps in landing configuration.
I do this every time. I also encourage my F/Os to the same. Great confidence builder. Shame the 3 for 1 plus "X" miles has gone out the window or most are just plane lazy resulting in the use of speed brakes.
I would love to see landings within 500' of the touchdown zone and on center line... but that's too much to hope for I guess.