The way I think of it, "SV_IsFrontFace.x" is the external (i.e., input layout) side, and "v1.x" is the internal (i.e., shader) side, and we're defining a mapping between them.
Yes, but which mask is the write mask and which one is the used mask? And why does it make sense to associate one to the external and one to the internal side? Having to choose I'd say that the write mask is the "external" one and the used mask is the "internal" one, but it's a weaker link than I'd put in a public interface.
Well, it's going to be available through the public API, right?
Yes, so it has to be properly specified; but for such an application I wouldn't be bothered if the specification is a bit rougher than usual (for example in requiring that a missing value is written as `-1`; or maybe something like `_` if you prefer).
Anyway, I can at least try to use the proper register name and see how complicated it becomes.