It's not too difficult to collect yeast from a carboy. You just need a good fermentation and the right sort of spoon. With a beer this big you will probably have to install a blowoff tube. The yeast will come up to the "neck" of the carboy, and spooning it shouldn't be impossible.
More over in "Brew Like a Monk" it is stated that westmalle doesn't reuse the yeast from the trippel. They reuse from blond a few times, a few times from the dubbel and uses the yeast collected from the dubbel to ferment the trippel. On the otherhand real breweries generate so much excess yeast that they can easily afford to do this.