I spoke to a Honda mechanic and he said gearbox in and out, 5 hours!
That's "by the book". Have a looky at the Haynes book to get the idea, most of the stuff they tell you to remove, you just unbolt from the gearbox and shove out of the way. With a lift and a bit of assistance, an hour or so, if you are geared up for it (no pun intended).
I imagine they split the box from the bell-housing then work on the bench
The bell housing is part of the gearbox end plate, so split that from the gearbox and it all comes apart. Even if there was some way to do this, I would imagine they would struggle to seal the joint again with it vertical. I still can't see how they can do it without taking the gearbox off.
If they do actually manage to do it in situ, I would love to watch! I might learn something...