Allow alternate staging area for backups

I have some accounts that are very large and thus when a backup is run, I get sent a diskwarn error for low disk space on my primary volume where the default staging area takes place. I'm backing up to a SD card and in turn uploading that via FTP once complete.

I can't continue down the path of being low on disk space as eventually I'm going to get to 100% and bad things will happen OR I'll need to move things to a bigger hard drive. Adding a bigger drive isn't the easiest solution as I'm using an expensive SSD and I'm in a co-location facility which doesn't give me immediate access to the server. I also figure I have plenty of space...just get low when the backup runs.

Seems like an easy thing to add to the backup options and would eliminate the issue altogether (letting me stage the backup on the SD card as well).