update - haven't been able to resolve problem above, but using an alternative function I was able to get a successful result:
int rc = 0;
TIRLOB_HANDLE* srclob = &##IN_SRC.WS_BLOB.GENERIC_4MB##;
TIRLOB_HANDLE* dstlob = &##OUT_DEST.WS_BLOB.GENERIC_4MB##;
rc = TIRLBAP(
dstlob,
4194304,
srclob);
##OUT_RC.CSA_GENERIC_WORK_SET.GENERIC_NUMERIC_11## = rc;