I have a ceiling that had some water damage from an upstairs bath tub . For a temporary fix I was planning to cut out a sheet of drywall to repair it, but was thinking when I go to remove the area to be patched, will the rest of the ceiling come down with it, or is plaster strong enough to withstand the removal of a section of it .
You can cut out a section of plaster/lathe fairly easily, so long as the keys are sound in that area. It might be a good idea to brace up the plaster around the hole you are planning to make in it (some scrap lumber...) Also, keep in mind that once plaster gets wet, it takes a LONG time to dry...so make sure you get all the damaged product out of there. I would scab in a piece of sheet rock that was a bit thinner than the plaster/lathe combo and then build it up to match the existing surface.