I might be missing your sarcasm, but this is a common approach for large scale builds. Virtual filesystems are used to provide a pre-computed tree hash as a xattr. In a more typical case, you can read the git tree hash.
Not sure it was meant as sarcasm really. I just think so many build (and other) problems could have been avoided it a file hash was available on every file by default.
In the current POSIX paradigm yes, it would be expensive. But if the hash was defined as the hash of fixed blocks, it wouldn't be expensive. The raciness depends, a lot, on the semantics we would define. (In the context of a build system, it's no different than that the file could get a new mtime after we read the mtime.)
