I use a RotoZip rotary tool (basically a Dremel on steriods) with the angle wheel attachment and a cutoff wheel. You can be fairly precise with this, as I have trimmed rough edges off demo'd plaster walls where they need to meet up with drywall. Lots of dust and noise, but it doesn't take forever and you can make a clean cut if you take your time. Cuts through plaster, lath, and wood with no problem. If I had to do what you are describing, I would reach for the RotoZip.
Another idea would be to use the RotoZip with the masonry cutting bit. Similar concept but with the cutoff wheel you are using it like a grinder, and with the masonry bit you are using it like a router.