When Pokébank comes out, try and get one on BW2 - You can replace HP Fire for the much better Heatwave. Because of that, I'd also consider HP Fighting. And maybe Nasty Plot to really smash walls:
+2 LO Modest Chatot (622 Atk) vs 4/252 Calm Blissey (652 HP / 405 SpD) using STAB Boomburst (140 Base Power): 300 - 354 (46.01% - 54.29%)
+2 LO Modest Chatot (622 Atk) vs 252/0 Bold Blissey (714 HP / 306 SpD) using STAB Boomburst (140 Base Power)399 - 469 (55.88% - 65.69%)
Thats a 2HKO with Stealth Rock and Spikes, or a 2HKO if its Bold. AND YOU'VE ONLY GOT 92 Base Special Attack :S. Scary.
That said, he's very fragile and very slow. I can see him getting a few opportunities to hurt slower teams, but faster will just seem him as an easy kill. Sticky Web support, and fully entry hazard support sounds really good so you can overcome his Speed problems a bit and punish switch ins a bit more. Paralysis support might also work too.
I also wouldn't rely on Nasty Plot too much - he doesn't have a great movepool lol. If their team is really slow, then you might be able to force something out.
Another idea I'd suggest would be using him alongside a Volt Turn combo. Scizor / Rotom-W sounds good, they can beat some of those Normal resists (Tyranitar and Aegislash mostly). That way, you'll have more opportunities to get him in. Otherwise you'll be relying on revenge kills and not much else.
Most of the time, he'll get one chance to do much damage - you've got to make it count. A team with lots of PKMN which hurt Ghost, Steel and Rock types would be good, so think Fire, Water Fighting, Ground and Darks (Dark types might want Pursuit too just for Ghosts). That way, you have lots of switch ins and ways to wear them down, and then open up wholes with Chatot's new best move.
Good luck with using Chatot - looks like he's finally got some potential :).