I could not get the ALTLIB command to work the way I thought it worked! Please ignore my earlier recommendation.
I was able to get this to work using BPXWDYN:
/* rexx */
"ALLOC F(NEWEXEC) DSN(MY.EXEC.PDS) SHR"
Call BPXWDYN "CONCAT DDLIST(SYSPROC,NEWEXEC) PERMC MSG(WTP)"
BPXWDYN is documented in "Using REXX and z/OS UNIX System Services", SA23-2283-01. Even though it is documented in a USS manual, it works anywhere in z/OS.