I have done something similar by adding an InnerContent inside another InnerContent. It was for adding quarterly reports, grouped by year.
Example:
InnerContent A "Year"
- InnerContent B "Quarter" (properties: "Report", "Presentation")
The result:
2011
- Quarter 1 (Q1-report.pdf, Q1-presentation.pdf)
- Quarter 2 (Q2-report.pdf, Q2-presentation.pdf)
- Quarter 3 (Q3-report.pdf, Q3-presentation.pdf)
- Quarter 4 (Q4-report.pdf, Q4-presentation.pdf)
2010
- Quarter 1 (Q1-report.pdf, Q1-presentation.pdf)
- Quarter 2 (Q2-report.pdf, Q2-presentation.pdf)
...
...
I don't know if this is what you're looking for. Hope it helps :)