Cached user credentials for accessing network shares would be a nice feature indeed, as there are like you say scenarios where you would want the backup process to be run by the local system account. However, the System volume information is restricted from the local administrator to manipulate for security reasons as it stores important information such as your volume shadow copies, EFS data, indexing and tracking log. However, you can grant the local account access to this folder. Beware that this implies a few security issues in the case your system gets infected with a virus, or the fact that you can simply delete information in it by misstake.
A sector imaging of your hardware does not take filesystem into account, therefore system volume information will still be backed up even if the backup job was run with the local administrator. Please correct me if R-Drive does not work this way, i know others do.