Develop an xls spreadsheet that will include all fields from the system. Create a monthly reminder that will prompt Curators when they login that it is time to run their local backup, and would they like to do that now. Agreeing to run the backup will download the backup to a designated folder on their computer or network. As an additional feature, a backup button should be available through the Manage dropdown so that a backup can be run at other times.
An XLS spreadsheet cannot be a good backup system : you will loose too many informations. For example, if you download a spreadsheet containing objects, you can miss entities details, and so on. It's the point with a relationship database...
If you want a good backup, you can do so with mysqldump through command line or with some GUI webbased like phpmyadmin or OS-based like Sequel Pro if you have a mac.
I can make a script to create a dump. The reminder can be a widget on dashboard.
Does that interest you ?
The idea here is to give users a file they can use to import to another system, kind of like AdLib's XML export format. A giant XML files with tags names for metadata elements is probably what is required. I agree that XLSX is not a good choice.