If you're in or near the U.S. Northeast, you might want to try for Lubavitcher Radio, a Jewish station on AM 1710. I've heard rumors that this guy is running some insane power level, on the order of 10 kW (!)
As for FMs... Engineer types on this board have confirmed that it's theoretically possible for signals of at least a few watts (say, 5 or 10) to get up into a duct, and there ARE stories of long-haul tropo and skip catches of low-powered signals (e.g. translators.) However, having some good friends who are FM pirates, I've noticed that tropo conditions actually hurt their normal fringe coverage, below 50 dBu or so. (Tropo does the same thing to KFAI, a 125-watt community station here in Minneapolis... yet DXers have reported it in Fargo on good openings.)
This suggests that, in order to hear an FM pirate via DX modes, you probably have to be a good ways more distant from the weak portions of the pirate signal's everyday coverage, yet not quite as far away as in a traditional tropo opening.