I rarely encounter plaster in the area where I am, but I feel your pain. I cut my teeth in this business doing service work in New Jersey. I dealt with a lot of plaster and lath in old houses. I don't have a simple solution for your problem, but I do have a few suggestions:
1. When cutting in a box, keep it as close as possible to the stud. They are harder to locate, but worth the extra effort. When cutting through the wooden lath, do it by hand with a coarse-toothed hacksaw blade, not a Sawzall or similar tool. Make the vertical cut to the lath on the side of the box FURTHEST from the stud first. The stud will provide more stability to the lath on the short piece left closest to it. Using a Roto-Zip tool is perhaps the best way to cut this hole.
2. Make sure that the box cutout includes one full lath width through the horizontal center and half a width above and below this full lath width.
3. Score the cutout with a SHARP knife and keep this scoring well within (1/8") of the actual necessary opening. If it's a tricky installation, then narrow it to 1/4" from the final opening.
4. Get the cable(s) fished in THEN prepare your final opening for the box. Again, a Roto-Zip tool will make the fine-tuning of the opening easier, but you'll need to leave the cable(s) inside the wall cavity while you do this. Don't let them slip away!
5. If you follow this procedure, all you should need to carry with you is a tube of basic toothpaste to cover basic cracks or chips that might occur during the final box placement that can't be covered by the plate.
True, it takes a LOT longer to cut the opening twice, but you will eventually get a feel for it and only need to cut once.
[This message has been edited by EV607797 (edited 02-01-2007).]
