I do not believe you can included property references like you want to in a software object (the RPM).
create a blpackage, and add the RPM to the BLPackage.
in an external command in the blpackage write in something like:
echo "BLAH BLAH" > ??TARGET.CUST_USER??
though, if you do this in the deploy job, as a 'post install' command in the job, i think TARGET is available. where were you trying to put your post command - in the RPM object or the Job ?