I think the key part of that article is:
One way to avoid the harsh commuting rule is to have a home office that qualifies as your principal place of business.
I'm not sure I would feel comfortable saying the place you set appointments and do administrative tasks (the home office) is the principle place of business, when all/most of the work for which you are paid is performed at a secondary location (the spa). It seems to me that the secondary location is the principle place of business. I think I would feel much more comfortable claiming mileage if your wife did the massages at client's homes, or many different spa's.
Certainly something you could look into further though. I think what you need to find is something specific to a professional who completes their work at a secondary location and has a home office where they do not meet clients, but perform administrative tasks. An independent contractor doctor maybe?