Compound turbos have been used a lot. They use the turbine shaft to mechanically drive the load, parallel with the flywheel. Lots of WWII airplane engines used them.
Until recently, Detroit Diesel 15L engines had a compound turbo that drove the gear train, but it seems they have ditched it now. There's probably a good reason for it.
EDIT: The DD16 (600 HP) still uses turbo compounding.
http://www.demanddetroit.com/engines/dd16/default.aspx