I pressume you made a typing mistake, mahogany is not used for fretboard, it`s quite soft and wouldn`t hold frets steady plus worn from strings.
mahogany neck/ebony fingerboard makes more sense, but more common and more usable combination in that case would be maple neck with rosewood board. Swamp Ash is similar to Alder so body is quite balanced anyway. Maple neck with rosewood board that`s the go. If you don`t like rosewood you can try goncalo alves, wenge or zicorte boards instead.
plus look, swamp ash is beautiful figured while mahogany is more boring looking, wouldn`t match