Other than skirts in whatever tartan and whatever length your wife finds fashionable and appropriate for the occasion--there is always the option of a tartan sash worn in a variety of ways. Scotty Thompson's "So Your Going to Wear the Kilt" does address this and I believe describes how to form a rosette from a tartan sash. (There are some who say that the sash is to be worn on the left shoulder save for chief's/chieftain's wives or ladies of nobility in their own right--as well as Scottish Country Dancers for that matter--but I've not seen hard evidence for this.)
Women have a lot of options for tartan because women's fashion varies and constantly changes much more than men's fashion does. So women can get away with whatever they like that is tasteful.

My wife doesn't usually wear much tartan save for one cocktail dress that is all tartan in a very small sett. (But then again, she's Irish and doesn't identify with tartan and such as a part of her heritage.)

But ultimately, since we are beyond Victorian era niceties, women don't have the same kind of "rigid ideas" as men tend to when regarding Highland dress.