Courtesy of the Epic Games forums

In games we use collisions to figure out what touches what.  It’s not a precision tactic, in fact, it’s one of the reasons you sometimes get hung up on “invisible” objects, or see someone’s arm stuck through a chair, but it IS one of those things that makes games possible.  It simplifies the interaction so that you can run a virtual world full of dozens of people and hundreds, even thousands of objects.

This bit about using ultrasound to create in-hair haptic feedback seems to me to be an ideal intersection.  For many applications, you don’t need precision, you don’t need to have a shape mapped out perfectly to get a reasonable interaction with it, so even with the very simple forms this ultrasound system can generate, you should be able to get some very good results.