I'm thinking of using ZFS to backups files to single drives that are going to be kept mostly offline. Does anyone know if the FreeBSD ZFS implementation still uses memory if a dedupped pool is exported?
Technically yes, practically no.
A minor amount of RAM will be used to keep track of the status of the pool, it is not removed from the kernel's data structure unless the drives are all removed. All ZFS file systems are unmounted however, so none of the file system caching (ARC/etc) is used.