This may be of no help because I can't speak to the symlink option (and I'm using the OS version) but you could just automate rsync's to the raid partition as part of your pre-backup scripting. Yes it has some IO tagged with it but so would having a symlink to a different fs especially if it's RAID5 (why not mirror? RAID5 is not ideal for heavy IO which is all backups are, is none of this being backed up to tape anywhere?).
At least this way you could tie it in to your existing backup schema ..... unless of course everyone is only doing local backup to disk using the Network version's internal backup operations
In that case this wouldn't buy you anything.
</2 cents>