The best way to do this would be to replace /var/airwave-backup folder with a symlink to the destination mount (this is the recommended method for users offloading their data to a SAN - though in the case of a SAN, we offload the entire /var directory). I suggest trying this in a lab environment to prevent any disruption to the production environment - to make sure it's accomplishing what you desire.
Logistically, what are the size of your regular backups? The tar process is known to take a while, but it's done in the background. The CPU portion is most likely from the tarring of the linear RRD data (used for graphs), you can probably get a good idea of how much work is being done by getting a count of files in /var/airwave/rrd (recursive count to include the subfolders = 'find . -type f | wc -l'). Also, 7.6 has been very stable, any plans to upgrade? Likewise, we've moved from Cent5.5 to Cent6.2 with plans to discontinue Cent5 support in 7.7 (http://community.arubanetworks.com/t5/AirWave/AMP-7-7-will-discontinue-support-of-CentOS-5-RHEL-5/m-p/63784#M1472).
The key is to test a backup to make sure that the symlink data persists. To do this, after you create the symlink change, run nightly_maintenance (the majority of nighly maintenance is run in the background, but a test run should either be done early before users amass, or later when users have gone for the day).