Hi,
You can use another NPE inbetween which has a reference to your MainEntity and to your DropDownFromExt.
Instead of using the MainEntity in a dataview you use the Connector entity:
I.e. create new Connector entity with reference to MainEntity and show that in a page.
When you trigger an action from that page you pass the Connector entity. From that you can retrieve your MainEntity and the selected entity and do whatever processing is needed.
regards, Fabian