Almost every small motor bike, scooter, moped, and off-road bike does this.
There are two windings on the alternator, the smaller one feeds a rectifier and charges the battery (which feeds indicators and brake light). The larger winding feeds directly to the headlight bulb only, feeding it with variable frequency AC and (to some limited extent) a variable voltage as well.
I had a Yamaha DT400 (400cc trial bike), it was my only means of transport, and I did a LOT of miles on it - I never had a headlight bulb fail.
I'm not going to get involved in AC vs DC, but it seems to make very little difference, and certainly for this application AC would seem by far the preferred method - if an odd bulb fails every year or two, so what?.