Does the breacan feile fall into any categories of use besides white tie functions and reenactment clothing?
I think only the latter. I did conduct a wedding in a Scottish castle some years ago where the ushers wore the breacan feile but they were members of a living history society that the groom was also involved with. Their costumes included 18 Century style dirks and basket-hilted broadswords.

More recently, at my son's wedding ceilidh, one guest did turn up wearing the breacan feile as day wear. In Scottish terms he was a giant of a man, although in the US his height would be less remarkable. He wore it well and gave the impression that he was very confident dressed in this way. If this thread is still running when I get back to the UK, I will ask my son if he is part of a reenactment society or simply favours this form kilt ... and post a picture if I can find one.