"Enter/Return" with, or without "Fn"
I guess since fn+return equals enter, the fn+enter-combination would equal fn+fn+return which does not make much sense, so my vote is: no, there is no app that can receive this combo. 

To prove, you'll need to read the key codes sent from keyboard to the system when it´s pressed (ie. Full Key Codes could do this for you)  - since I don´t have a keyboard with a physical enter I unfortunately can´t do that.