Yes in general most bottle conditioned beers will age well, particularly if they are strong. Although Geueze tends not to be strong, I think the reason behind its aging is that the intense sharpness of young lambic beer mellows and gains complexity with time, that's why geueze is a blend of young and aged lambic beer, I think the old stuff adds complexity and the young stuff gives it a bit of a kick. I haven't had any significantly aged lambics, although I think in general it is only bottled after it is a number of years old already.
I'm not sure about westmalle, I can only presume it will benefit with age. I have heard that Rochfort ages very well. (in CAMRA's good belgian beer guide and also in "brew like a monk")