There's no pre-made switch/flag that's going to do this AFAIK. If you know Perl well enough, take a stab at /scripts/cpbackup. Otherwise, ask your provider to modify it to implement that feature if your server is managed. It shouldn't take more than 5 lines or so.
There's no option for that either. You'll need to modify /scripts/pkgacct in this case to implement the feature. To hide the backup icon altogether when/if the /home directory is larger than 1G is going to take some work too. You could change /var/cpanel/cpanel.config and lower loadthreshold so any ongoing backup is forced to wait until the load goes back down.