You're probably better off doing a pre-process on your file using unix.

eg. let's say you have this in your original ' unload to /tmp/jon/x.unl select * from customers where creation_date >= :creation_date '

cat [original file] | sed 's/:creation_date/01-04-09/g' > /tmp/jon/tmp.sql

dbaccess dbname /tmp/jon/tmp.sql

rm -f /tmp/jon/tmp.sql