I learned how to do this from the fine folks over at AirDev and it works wonderfully. Screenshots are from one of my apps.
Make the reusable element’s type of content the same as the content of the repeating group.
Do the same for the focus group and set its data source as Parent Group’s Thing.
Then when you place the reusable inside the cell of the repeating group, set it’s data source as Current Cell’s Thing or Parent Group’s Thing if inside a group.
BadaBing, here’s what you get.
Just remember you will have to create any elements this menu needs to trigger inside the reusable element as you can’t reference elements on the page.
Hope it helps!