Possibly. I have found such walls that would not support the box. Removed a few of the exterior clapboards and nailed in a piece of 2 x 4 between the studs to support the box. Not fast nor low cost.

The standard inexpensive old-work boxes have a piece that extends behind the wall and then clamps down and holds the box as you screw it in place. However, the clamp piece is usually not big enough to go over thick plaster and lathe. Even if you could get it to clamp, the box & wall may not be strong enough to hold a sconce if its fairly heavy.

If you can locate the studs and it works with where you want the sconce, you can make the box cutout right next to the stud and screw through the box side into the stud. This will hold a metal box very firmly. Even if the box is not right next to the stud, with patience you can often insert a couple of small 2x4 blocks or cut plywood, and attach them to the stud so you have a place to screw on the box.

Doing this type of work is more tedious and time consuming than it is difficult. Its a good skill to learn yourself, since its better to pay the electrician for actual electrical work, while you work with your old house and try to figure out mounting points for devices and paths to fish the wires. Most electricians will also use the fastest method of fishing wires and installing devices, while you may have the time to investigate more and use the method that requires the least repair.

Bruce

Not the answer I was hoping for...but thanks so much
bus_driver and Bruce! I'm definitely not a DIY type,
so sconces may not be practical after all.

just make sure the box cut out covers 3 lath boards..
this way it have some thing to hold on too..
it might look like it will just take out 2 boards..but don't do it..it doesn't work.
move the box up or down so you have to cut 3 lath board..one board completely and notch the upper and lower..

good luck..

-dkenny