I just added the folder to show that you can organize your data in subject hierarchy folders (same as in Data module). You can use the folders any way you want. @cpinter works on allowing folders to override display properties of all nodes within it (e.g., you can temporarily show/hide or override color of all markup nodes within a folder).
Is there a good approach/format that you would be confident to recommend?
Exporting measurements to a table node would not be difficult. There could be an “export” section in markups module GUI to create a table node from selected measurements from all selected markups.
Tables would be regular table nodes, so you could load them again into the scene. However, maintaining two-way links robustly between some original data (e.g., point sets) and measurements on them would require UUIDs and cross-references. Instead of reinventing the wheel, it could make sense to use DICOM for this.