I store part of my library on a NAS, and some on my local.
Is there any way to hide the network files that are unavailable while I’m working away from home, just some I’m left with the local files and any “local copies”?
I don’t think that’s possible. I’d move the whole collection to your NAS then use the local copies feature.
Otherwise you could have two libraries.
If you use the folder filter you’ll see that the not connected folder names are striked-through. And to select only pictures with local folder you can select the corresponding folder from here.