You can hook up an NES to a FDS. You just need a HoneyBee adapter and a top loading NES.
https://www.youtube.com/watch?v=8cnJZXhpHsU
Right, the connector is a RAM cart that the FDS writes data to that is then read by the system, so this makes sense. There's no use of the expansion slots or anything like that. The additional sound hardware is in the FDS itself and works in conjunction with the Famicom.
Meathooks still managed to get something wrong in that video. He says "4 D batteries" as he opens the system to clearly show 6 C batteries inside. Weird.