I may have buried the lead with that title. Project Fi now supports data only devices, but not just that. Project Fi now supports non-Nexus devices. Not only that, but non-Android devices.
Of course, you need to have a cellular tablet to even consider this, which no one really has. I do, but most people just use their phone as a hot spot.
The list of supported devices is as follows: Nexus 7, Nexus 9, Samsung Galaxy Tab S, iPad Air 2, and iPad Mini 4.