These seem difficult. Gluttony could be crudely represented by serving too much of something rich, but that's wasteful. (I know, it's appropriate, but I dislike waste.) I'd rather do something more subtle. A dish that would offend the politically correct would perhaps be ideal, but there are limits to what can be done practically. Perhaps something wasteful in the sense that most of it has to be discarded to get at what's good. Artichokes might be good, but it would be difficult to get people to eat them - actually, that might be clever.
Greed is even more difficult, it seems to me. I mentioned in an earlier post shorting each table one serving, that's subtle, but it also needs to be a representative item, I think. This requires some thought. I was also considering something that appeared to be expensive, or actually is expensive or perhaps gilded with something that appears to be gold or silver. Caviar would be perfect, but I'm not sure how medieval, and the cost would be prohibitive. Perhaps a faux caviar of some kind. Hmm.