Hi,
Some more ideas regarding attributes for lines:
* Use color coding: Have a special color that is translated later on
to the right description,
I think you diagram is using this already.
* Use something like UML Aggregation with text color set to the
background color.
Attached is the simplest List export filter I can think of in Python.
It exports the text values of all standard text objects. To try it
out, just copy it e.g. to a folder ~/.dia/python
I hope that your Dia version is built with Python support.