Is there wisdom which actually cannot be fully expressed except in poetry or literature or art? Or is addressing philosophical questions in such an "artistic" manner just a way of jazzing up an argument which could have stood cut-and-dried, anyway? Is there anything Homer could teach us which Plato could not?

It really depends upon what you mean by "wisdom." The way you put it, it would appear that you mean the same thing by "wisdom" as some information or statement that might otherwise be produced by philosophical argumentation. I am not inclined to call such things "wisdom" at all, though they may be produced by wisdom of a certain sort. Rather, I am inclined to think of wisdom as a kind of virtue or excellence of character--one which is characterized by intellectual skill, perhaps, and also by practical intelligence. If this is right, then wisdom is not simply some information that can be concluded via argumentation or for that matter enunciated in poetry or other art forms. One with this virtue, however, might reveal or enact his or her possession of such wisdom in the things they might say or write, and also in the appropriateness and effectiveness of how they say or write what they do. It could well be that the very best way for a wise person to communicate something of importance to a target...