So I figured out the answer is that yes, belgian corks will hold pressure in a standard champagne bottle. I've got two points of evidence to support this:
I purchased a bottle of Westmalle in a 750 mL champagne style bottle with a belgian cork. I measured the opening with calipers and found it was the same diameter as several different kinds of commercial champagne bottles I've saved from New Years. Since the beer was obviously carbonated using a belgian cork, I could have stopped right there. But since I've got too much time on my hands right now, I took 1 L of water and rehydrated a pack of pasteur champagne. I then added enough sugar to prime the liter for 6.2 volumes. 750 mL of this "delicious brew" went into a champagne bottle from Mumm with a 25 mm belgian cork and wire cage. I let this ferment out over about a week, then opened it, foolishly, at room temp. This resulted in a nice yeast-water gyser in my kitchen. The water was appropriately spritzy, and tasted exactly like you can imagine a warm slurry of yeast and water would taste like.