I'm thinking about having this done myself, though at the rate I'm dragging my feet it might be a project for next summer. In any case, I was leaning towards linen but I'll be following this thread closely to see what others recommend.

I wear a jacket (not with the kilt) every Sunday through the summer to Mass, no matter the heat. I'm a cantor at our parish and I just feel underdressed at the ambo chanting the psalm otherwise. I have a couple jackets I favor for summer wear. One is a lighter weight wool (which actually does me fine year round), but my favorite for summer is a light weight silk/wool blend. It's 55/45.