How big are those holes going to be?

DH has usually straddled a piece of furring strip/lathe in the opening, with a wire, pencil, and gorilla glue. If small, he'll patch it with joint compound. If it's larger he uses a mesh drywall patch.

They will all be 6'' holes. What's DH?

Just realized that your question was about the hole to fish wiring. It'll be one long hole (about 3'' wide and 7 feet long) that will run across joists.

Rocklath was a material similar to drywall, but prepped to take basecoat plaster. The three-layer system was every bit as good as any type of residential plasterwork.

I think the paper material under the finish paint is probably wallpaper.

Just fit the pieces that you cut out back in and use shims like in the picture below to support them.

Then tape and mud.

BTW - Big holes are just as easy to patch as small holes. So there's no real point in trying to keep your cut channel to just 3" wide. If it's a little wider, it'll be easier to fit the shims in.

DH = Dear Husband (or dear handyman ;))

Thanks to the picture above - that looks like what ours look like. Sometimes for whatever reason it couldn't be screwed in, so he'd use some pencil and twist a wire behind it to hold the strip while the lathe dries.

I have also had good luck with using these clips for smaller patches. You will need to predrill the holes in the plaster so it does not fracture, and you also need to open the clips up so they fit around the thickness of the plaster (its probably about 1" thick). http://www.homedepot.com/p/Wal-Board-Tools-Drywall-Repair-Clip-6-Pack-54-014/202015408 . Once you get the patch screwed to the clips (or to wood backing), you then use standard drywall tape and plaster to patch the edges.

When you predrill holes in plaster I have found that I need to drill a smaller diameter hole for the shaft of the drywall screw, and then drill a larger diameter shallow dimple for the screw head to recess into. If you try to just screw it down tight like you do with drywall, the stiff plaster finish will crack instead of dimpling in like drywall does.

Bruce