In summer 2005 I was the orchestra director for MTG's Sweeney Todd. I reduced the orchestration to only five instruments: flute, clarinet, violin, cello, and keyboard (aka Pierrot Ensemble). This enabled us to fit on a tiny stage with a large cast, and not drown them out (they were not amplified). It also allowed me to use exactly the same excellent ensemble for every single performance of a 3-weekend run, which would not have been possible even with the "official" reduced orchestration.
The keyboard used only real keyboard sounds (piano, pipe organ, and of course harmonium). The pipe organ helped to make up for the lack of brass in the climactic scenes.
The same orchestration was (mostly) used by conductor/pianist Maria Duaime for Metro Stage Company's 2009 production.
If you're doing Sweeney Todd in a small space and are interested in using this reduction, please email me.