It looks like shortly after users found out that you could add every NES ROM to the NES Classic that we now have a RetroArch port that can be installed as well. This has been confirmed on the RetroArch Twitter as working. 

Somebody managed to put RetroArch on a NES Classic Edition! https://www.youtube.com/watch?v=Uj4_clJg_kA&t=201s

This build of RetroArch is a custom build by MadMonkey he has done all the leg work by writing up a post on how to install RetroArch. So if your looking to have some more fun with your NES Classic make sure to check out the link HERE!