what sort of sedation?
i've used sedalin and intraveneous (sp?!) sedation for my mare for clipping and vet didn't say anything about starving her before either option... just about being careful afterwards with hay etc for a few hours as they sometimes forget to chew when they are heavily under!
Sedalin does work better on an empty stomach, but shouldn't matter either way for iv.
Don't assume that the sedation will completely knock them out as some horses still fight it -my lad still managed to kick my vet over whilst heavily sedated for a hock injection....!