Alright, per network storage on the nVidia Shield, I don't have good news. I'm not sure if that will actually work to host ROM files. Unfortunately, when nVidia implemented it, they did it in a proprietary manner that does not appear to let applications use it via the proper Android storage APIs.
I haven't found details on how to access it properly in LaunchBox, and I haven't found any emulators that seem to be working to load up ROMs from it either, so I'm guessing if the emulators can't see the network paths, then it won't matter if LaunchBox can import them, because you won't be able to launch them. If anyone has any more details on this from a programming perspective, do let me know. But what I've seen so far suggests that it's not likely possible, sadly.