How can I perform this database backup/sanitization task?
Setup a slave.

Once everything is sync'd up and good. 


Stop the slave (sql thread).  
mysqldump slave data to orig.sql
run your queries to modify the data
mysqldump again to dev.sql
Restore the data using orig.sql
start slave to resume collection from the master.


This will take some time with a large db of course but it seems pretty straight forward to me.  It won't lock your master cluster at all (at least until you create the slave).  If you are modifying only a few tables you can tweak the mysqldump.

You can cron the operation and upload it to s3 or something for the devs.  Use some fast SSDs.