I know you say the project does not use DataGroup but I have a suggestion that uses the DG to apply labels that would not conflict with the established workflow.
If you create very simple Catalog Items in the DG that contain the properties that you wish to label (using either properties that already exist in the DG or custom properties that you define) but are not associated with any graphics you can apply these Catalog Items as Instance Data to your non-DG elements.
You can then create DG Annotation Cells that include the appropriate fields from your Catalog Items and then apply those cells manually or automatically via Drawing Rules.
This Widget Item I created a while ago suggests what might be possible, I used the existing ID properties and added a property group called Things. Does that look like it might be useful?
