Okay, so it's not a perfect match yet, but apparently Korean Advanced Institute for Science and Technology has made a Robot Chair that looks surprisingly similar to the Throne of Judgement that the 40K Witch Hunters can field.
All they need is a bit of extra armour, some heavy guns and and old guy sat in it and they're set Oh, and they'll need to give it a real generator too, instead of running it off the mains.
The page also includes a little YouTube video of someone in it and moving it around.