There are a number of places in the ICU code that fall back to a hardcoded string in the case where there is no data. Typically those match (or should match) CLDR root, but it is easy for them to drift.
Rather than have these be hard-coded, I propose having:
a) a low-level file with some static const in it
b) a tool that generates those from CLDR data (and overrides/additions as necessary).