It's not hard. Symlink your pool (commonly /var/lib/BackupPC) to your mounted NFS path (or change the TopDir). I did it this way for a few years.
That being said (and with the repeated statements of "storage is cheap"), I never exceeded 3-4 MBps on a backup run. In the end, it was better to slap a large drive into a machine and keep the pool local. Hey, any number of people might've said that.
-Kris