If you know what you are doing, you can program the Harmony to do the 30 second skip forward and 10 second skip backward thing. I am sure that it's already in the Harmony Remote software for the Apple TV though. It just might not be used by default. Try going into the settings and assigning the additional buttons and you should find it.
So I finally fixed this last night.
I first tried going into the Harmony settings for the Apple TV preset and moving buttons around. There is indeed a "skip ahead" and "skip back" button, which I assigned to my ff and rw buttons, but when I actually used it, it jumped to the chapter markers, not 30 seconds. (There also is a "next chapter" and "previous chapter" option, which I assigned to different buttons. They did the exact same thing: jumped to chapter markers.)
So, as a workaround, and based on suggestions I'd seen elsewhere, I selected Tivo Premiere as the preset for my Harmony remote (even thought I don't have a Tivo), just so I could get a remote with all the buttons I wanted emitting some infrared signal. Then, I used Apple TV's "learn remote" feature to assign functions to the buttons I wanted. This made the remote work in exactly the same way as it had before, when I was using the Apple TV Harmony preset, with the exception that this time, the skip forward and skip back buttons actually did do the 30-second skip forward and 10-second skip back. Bit of a kluge, but it works great.
Not sure why the Harmony Apple TV preset skip forward and skip back buttons don't do the same thing as the skip forward/back assignment in Apple TV's learn remote function, but oh well.
(Also note I'm sure you could use a Harmony preset other than Tivo premiere. I just chose one that I thought would provide enough button functions to make the Harmony buttons I wanted to use active. If you leave a Harmony button unassigned, it won't emit an infrared signal.)