IanR,
One handy trick I have found works well for cutting in old-work boxes is to cut the plaster with a 2" grinding wheel chucked up in a drill motor or other rotary tool. The lath and plaster does not seem to break up as badly. This works pretty well with tile and brick also. You would want to have a vaccuum running to catch the dust.
Or else use a variable speed jigsaw to cut a receptacle into the baseboard using slow speed so as not to shake the wall (orienting the box horizontally instead of vertically). A dark brown receptacle and cover blends with the wood.
