I know that Hijack has a shutdown timer, and I use it regularly. But I think that this feature would be better implemented in the player, as it could then use its knowledge of track lengths to power down on the nearest track boundary.

(There's also a problem with Hijack's method, in that it injects a long press of the Source/Power button - if the player is away, blocked on disk access, it sees only a short press, and spends the rest of the night on Aux input. Since the kernel knows when the player reads the button code, can it use the read to know how long to hold the button?)