Data merge labels with text in multiple orientations

I’m trying to create call number flags from a library catalog spreadsheet. I want them to be narrow strips that can be inserted into books like bookmarks. The call number should be parallel to the short edge, and the title and author should be parallel to the long edge. I can’t find any way to do this with a document created by the label wizard.

I have tried adding vertical text boxes, but I can’t add data source field references to them. I’ve tried using tables, but there doesn’t seem to be any way to change text orientation in a table cell.

I don’t think “labels with some text at 90 degrees to the rest” is a very odd use case, as this is extremely common in label layout. What am I missing?