I don't know if this tool would work effectively on lath & plaster, but Makita make a twin-diamond blade chaser ( 1/4"- 5/4" wide, 1/4-7/4" deep). On Makita's UK website the tool number is 'SG150' (it's 110v), but I was unable to find it on the US website. I've also seen a smaller version, Makita 240v 'SG1250' , on the Screwfix UK website. Other makers should have similar tools.
Alan


Wood work but can't!