Answer: using STDIN to point to a file with a "y" in it (no quotes) and nothing else.
For example:
//STEP1 EXEC PGM=BPXBATCH,
// PARM='SH /usr/lpp/ars/V8R4M1/bin/arsdb -I ARCH841 -vd ARSSYS'
//STEPLIB DD DISP=SHR,DSN=MY.SARSLOAD
// DD DISP=SHR,DSN=MY.SDSNEXIT
// DD DISP=SHR,DSN=MY.SDSNLOAD
//STDIN DD PATH='/ars/tmp/yes',PATHOPTS=(ORDONLY)
//STDERR DD SYSOUT=*
//STDOUT DD SYSOUT=*
//STDENV DD *
DSNAOINI=/usr/lpp/ars/V8R4M1/config/cli.ini.prod
NLSPATH=/usr/lib/nls/msg/%L/%N:/usr/lib/nls/msg/%L/%N.cat
RESOLVER_TRACE=STDERR
