I'm sure a proper solution will arrive later.
You could exclude all but "999" to one file. Then use JOINKEYS with that file as the second against the original file. Take those that are unmatched and you have your data.
I'm thinking something like the following:
//DROP999 EXEC PGM=SORT
//SYSOUT DD SYSOUT=*
//INA DD *
//INB DD SYSOUT=*
//SYMNAMES DD *
//SORTOUT DD SYSOUT=*
//SYSIN DD *
//JNF2CNTL DD *
Unfortunately I can't test it, and I'm only learning about DFSORT as well. I have hopes that it would work, but no idea or if optimal. My first JOINKEYS attempt