After a little experimenting, I hit on a good way to drill when not aligned with studs or joists. Ordinary twist drill bits don't work well, masonry bits are recommended. I found out that a roto-zip tile bit chucked into a Dremel tool does wonders. It grinds a nice neat hole in a hurry, with almost no pressure – just what I need. Then I use a small diameter drill or router bit to run a pilot hole in the lath behind it (again, with minimal pressure). Then I reach for my box of the McFeely's square drive (Robertson) screws that I've mentioned before. They have a sharp point which will solidly engage the pilot hole without pushing, and the square drive means I don't have to use pressure to keep the bit engaged. And that makes for easy, safe attachment to plaster walls.