I hope your hierarchy has no more than two levels or the performance (and maintainability) will suffer.
Basically you will need a collection widget inside of a collection widget:
- Collection Widget (Parent)
- Either content or another collection widget
- [...]
The mashup that represents each row, must evaluate a mashup parameter to either display the dropdown (or whatever you want to show) or another collection widget which has essentially the same setup as the parent. I think I would just use the "Contained Mashup" widget for this. In theory you can create as many layers as you want... but the data handling is no fun.
