THere really is no easy way to do this with the default scheduler. If you are going to set a different permission level for backups on different dats, you would have to right-click on each date, pick Properties, and set teh protection for each date
As of DP 8.1, there is new functionality called Advanced Scheduler that looks like you would be able to do what you want. If you have DP 8.1 or higher installed, in the Backup specification, click on Actions, and pick Advanced Scheduler
The new DP 9 Admin Guide has Advanced scheduler information. For 8.1, click on Help -> Help Topics, and type Advanced Scheduler in the search window