There is a product from Avermedia called Avermedia USB Presenter-MV which is available from Mac Warehouse for about $100.
It does mirroring over your USB. But it is not real high quality. It updates the TV about once every two seconds and the quality is medium at best.
If all you're doing is a slide show with something like Powerpoint, you could get away with it, otherwise I think your best bet is to have a computer with a real video out that travels with your projector.
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