I think WLP550 would probably be fine, since it seems to have a pretty similar flavor profile to the WLP400. The WLP500 would be way too fruity, but yeah, I think WLP550 would be okay. Of course, you might even be able to go the other way and use the WLP400 for a regular belgian ale.
EDIT: And in fact, the chart says that's a better fit. Now I'm considering pitching something on top of my wit yeast cake.
WORLD DOMINATION BEER
#48 Black Helicopter Porter #49 Pikemen Porter
#50 Imperious IPA
#51 Razz Buried