I have created several scripts using the VSSBackupHelper and DiskShadow CLI programs.
The problem from the GUI perspective is that you have either run them prior to running the GUI, inside of a container so that the VSS drive is visible to use as a target, or go back and edit the backup files manually replacing the drive letter with the VSS drive, or use the SUBST command to setup the target and then close the application and script the backup from the CLI.
Does this make any sense? If we could tell the target to 'override and use this drive letter' or 'assume this vss drive letter'. I love the way it validates the existence of the source, but when dealing with VSS stuff, it gets tricky.