... and they're still compressable

As a corollary to the last entry, an orthogonal tangent basis is commonly compressed by storing the normal and one of the texture axis vectors, along with a "handedness" multiplier which is either -1 or 1. The second texture axis is regenerated by taking the cross product of the normal and the stored axis, and multiplying it by the handedness.

The method I proposed was faulted for breaking this scheme, but there's no break at all. Since the two texture axes are on the triangle plane, and the normal is perpendicular, you can use the same compression scheme by simply storing the two texture axis vectors, and regenerating the normal by taking the cross product of them, multiplying it by a handedness multiplier, and normalizing it.

This does not address mirroring concerns if you use my "snap-to-normal" recommendation, though you could detect those cases in a vertex shader by using special handedness values.