I wanted to use my Dendy Junior pad on the AVS via the expansion port in the back, and it works just fine for the menus, but it won't work for any of the actual games because the system recognizes anything in the expansion port as a player 2 controller. Is there some way to configure it to recognize controllers in the expansion port as player 1?
Using firmware v1.40b4 if that makes any difference.

Not the exact answer you are looking for, but should work. I wanted to use a Famicom expansion port controller on a nes. Ended up making an adapter to do so. Cheap to do it. You need a male Db 15 sub-d and a nes controller extension cord. Here is the link to the post about it. You need to follow the pin out diagram and ignore controller 2 info.

https://www.instagram.com/p/BkY6M...

Edit:
Deleted pinout diagram image, it says only for use on Famicom world, link to message thread on Famicom world that has the pinout diagram . They are wiring for the reverse of what you are looking, but it works both ways, just ignore controller 2 wiring. 

https://www.famicomworld.com/foru...#msg55602.

I appreciate the help, but I don't have any soldering experience and would prefer not to attempt this myself. Definitely not something I've ever done before.

There is no soldering involved. Cut the the extension cord in half, split the wires, follow the pin out, insert the wires, close the shell, and you are done.

Just putting it out there, I know it is not for everyone. I believe I found someone making them for sale. The price was too steep for me, $50 I think, but ended up making it for $15(?).