Ah, CMcG, isn't it interesting to consider things, like the Tartan Army putting on kilts for the game, to be kilt rituals? That is what I'm getting at with the question. The address to the haggis, if I remember what it is called, that seems to be done at Burns nights was one ritual that came to mind. On the other hand, hiking in a kilt of some sort seems to be another ritual of manhood.

Chirs, Osprey has a few books, one of which I have read, that show Roman garb and armor. I think the armor is skirt like, but a tunic was usually worn under all that. In the beginning of a book by a different publisher on Medieval armor, a Roman tunic with a slit in the back is described for riding a horse. Also Xenophon talks about arranging one's skirts when getting on a horse in, On Horsemanship, if I remember correctly.
I'm just listing this off the top of my head, though I could look up the titles and authors of these books. I'm sure you could easily find much better proof in case he challenges you. You never know...