I have worked with read only data, particularly read only databases all the time and my concerns are not so much from the backup side of things, but more from what I can't do once I make that data Read Only. So, you need to make you sure complete these things before you set the database to READ ONLY mode, because once you set it, then:

•Statistics will not be automatically updated (nor required) and you would not be able to update statistics of a READ ONLY database

•READ ONLY databases will not shrink automatically or manually

•You will not be able to create indexes

•You will not be able to defragment indexes of a READ ONLY database

•READ ONLY databases will not allow you to add any extended properties on any of its objects

•Permissions may not be edited and users may not be added or removed from a READ ONLY database.

Backing up read only data is only a small part of a much bigger picture. 😀