Mediaplayer can make playlists, but I don't think it can auto-generate one from a directory.

Here's a simpler version, though:
Not quite equivalent to yours, as this wipes the playlist if it already exists; I think this is more useful, but if you wanted it to append, you can change > to >>, of course.

Edit: Oh, by the way, ditch media player, use mpd and powerlaunch, and the N800's one-handed, too. Or write your own non-powerlaunch app (could even live in a terminal), just to intercept keypresses and call mpc, thus avoiding the complex generality of powerlaunch.