Misty,as I understand it (and I'm working from wetware memory here, not from wikipedia or google)the fleur-de-lis being the crest of the royal Bourbon family of France (including Louis XIV), it was associated with Louisiana (note the name) as a French colony and, since the Louisiana Purchase, has remained a symbol of French heritage in that state but especially in New Orleans. Hence the local association with Mardis Gras.
I'm not a real art student, and I don't know history or theory. I'm sure that you'll use your own analysis, and I guess you don't want to use just that. Your research seems like it'll have to be based on sadly indepth sources. You have the time-frame of its usage, and you're finding great ways that it has been used. I'm assuming that you should look for significant artistic and literary works that would be good candidates and see if you can find important uses.The harder part is trying to argue why you think it was significant, but perhaps you have design and art books that can support your theories. I don't really know how this is going to go down in your actual project, so I hope you find something.
Misty,the symbol is so common and so powerful that it begs for a good analysis. I very much like your "sighting journal" idea. And as you follow up on the history (certainly you'll find good articles/books on this, even if you haven't yet done so) you ought to have a rich set of facts and ideas to work from.