[einniv]

1) For the time being use local ROMs and assets.

2) Alternatively, you can use tools like autofs (some help here and here) to automatically mount remotes when they are needed. In my setup, I keep ROMs and artwork in a Synology NAS and then use autofs to mount the remote filesystems using SSH on demand. SSH is configured to auto log in with a SSH key pair. It works like a charm. I'm not sure if autofs (or a similar utility) is available for Android, though.

3) Kodi Leia will include Retroplayer and I believe it will be able to launch ROMs from any remote Kodi supports (however, it is still not clear). In any case, soon AEL will stop using the Pyhton filesystem functions and move to Kodi VFS filesystem functions which will enable true ROMs and artwork in remotes. However, if you want to use external emulators ROMs in remotes won't work so the most flexible option if you want to have ROMs in your NAS is to mount the remote filesystem with an external tool like autofs. 

  For most Android devices 2) , or something like it (mounting samba shares), will be an option if the user is rooted and has installed an OS replacement. For me I am using an Amazon Fire Stick which as of now cannot be rooted (though earlier versions could be).

I did find a way forward that falls under option 1) but is still an improvement. I thought I'd leave it here in case other fire stick users wander by. It appears that on 2nd generation fire sticks you can attach a usb thumb drive by using what is called an on-the-go usb connector. Details can be found here. Then from within Kodi you can set up a source in file manager by browsing to "Root filesystem"->"storage"->"usbotg". Once doing that the thumb drive you attach to the cable should look like any other local file to Kodi and , I should expect, RetroArch. Keep in mind I haven't actually tried it yet but by all reports this should work just fine.