As long as your yeast is healthy and abundant and you're not cold-crashing the beer as soon as fermentation tails off, you'll be fine. I've fermented with a number of Saison and Belgian strains (though I can't remember which off the top of my head) and I've never had any problems with diacetyl. Also, basically any strain that's known to produce diacetyl in over-abundance will say so on its spec sheet.
The elevated temperature many Belgians ferment at is actually a boon to yeast's reduction of diacetyl to flavor-neutral compounds after primary. Also, most Belgians are medium to medium-low flocculators (highly flocculent yeast tends to leave more diacetyl around). More yeast in suspension = more power to reduce diacetyl post-fermentation. To be safe, there's really no reason not to just let your beers sit for a week at primary temperature after fermentation is done to make extra sure all the D is gone. A small, virtually risk-free step you can take to ensure good beer.
As far as I'm concerned the 'No diacetyl' comment should be applied universally (if you read a lot of BJCP guidelines, you'll see it so many times you'll stop even noticing it). Luckily, provided you follow the general practices of good fermentation, it's really one of the easiest off-flavors to avoid with pretty much any ale yeast.