Since you need to set the RC, that is a good place to start. You can only set the RC if SORTOUT or an OUTFIL dataset contains no records, so that is what you have tp get to.
You don't read records yourelf in SORT.
The only time you know the source of a record is with JOINKEYS.
So, use JOINKEYS, with STOPAFT=1 in the JNFnCNTL files.
Use the date for JOINKEYS.
OPTION COPY,NULLOUT=see what you can have..
Your idea, with the NULLOUT/NULLOFL would generally work with INCLUDE/OMIT to get just the headers, but you'd have to pass all the data. Set a sequence number. Use OUTFIL to OMIT sequence number of 1.