Looks good!

Perhaps if --inputconfig flag is not set, it could check $OCIO? Might be a convenient way to verify the env is setup correctly, and may answer many "why doesn't OCIO work!" questions. Something like:

$ ociocheck
Checking $OCIO environment variable
Loading /wrong/path/to/config.ocio
Error: File does not exist
$ ociocheck -i /correct/path/to/config.ocio
Loading /correct/path/to/config.ocio ...

Could be useful when there env is less simple, such as inside applications that might be launched from wrapper scripts (e.g run subprocess.Popen("ociocheck") in Nuke's script editor)

Also, I wonder if requiring boost for the config-validation utility is reasonable? While I'm not fussed about the dependency, it seems like ociocheck is something that should always be available, even if built without the optional Boost. I guess the alternatives is to use something standalone like optparse.h, or copy that bit of boost?