I've been playing around a bit with the TS-E 90 + 13+21+31mm Extension + 2x Extender and I got modestly sharp results at 5° tilt (more would have been unsharp) and could take a photo from a 10 degree angle. That was just enough to keep the lens (without hood) out of the reflection of a flat surface.
The reason for the low angle at the high tilt is in the short focusing distance (relative to the focal length of the lens, so a longer lens will not help here). When the Scheimpflug angle is 90° for infinity focus it's almost flat for a macro photo.
It would be great to take a 1:1 macro photo of a flat surface from a 45° angle but that would require much more tilt and the current lenses become very unsharp at large tilts when used with extension tubes.
So maybe a special lens design with a fixed large tilt would be possible but that would only be useful in a few macro scenarios. Still i'm curious what the macro tilt design is they may present.