Following up on this instead of posting my own thread, as this is my question too. Except I ordered and just received the Retron HD and it is not working.
I have the Famicom N8, so with the Retron HD I'm using the Gyromite adapter.
With HDMI cables, I get absolutely nothing when using the Famicom Everdrive. And I tried all 4 possible combos to turn the cartridge/adapter just to be safe.
With AV Cables, I get a white screen.
So, is it definitive that there is no way to use the N8 with the Retron HD? I had seen some "yes" and some "no" from reviews going in, so I purchased it. But this is really the only thing I was interested in, using it with an Everdrive.