Yes, it works.
My existing rear derailleur (Shimano Alivio M410) was suitable to work with the largest 34T cog. The B-screw (angle adjustment) on the derailleur will need to be tweaked to prevent rubbing of the cassette/jockey wheel/chain. In my case the B-screw was turned almost all the way in, but it works great. The change between the 2nd-largest and the 'Mega' cog is actually smoother than I expected, despite the big jump.
I have fairly small front chainrings, so the tensioning of the chain was not an issue. (I was fitting a new chain at the time anyway)