I had to go through the same experience in the hallway of the upstairs of our 1924 house when we installed an attic hatch staircase a couple of years ago. I employ the same method when I have cut the rest of the holes in my plaster walls from outlet holes to big square holes for new ventilation and a medicine cabinet.
I don't know of an absolute PERFECT way of cutting into old plaster without breaking away surrounding plaster. I had a heck of a time with the medicine cabinet and broke away so much surround plaster that I had to install pieces of drywall to fill in the gaps.
However, that was just that one case. Everybody's house is different and all old plaster is not going to behave the same way.
So the way I cut into plaster is to place masking tape over the area I wish to cut my hole (assuming it is going to be a big hole) that will have to be rectangular to admit a box. I draw lines on the masking tape where I wish to make the cut. I cut into the lines with a mat knife as deep as I can and remove the top portion of plaster in a hopefully neat square. I then chip out the rest of the hole with the chipping hammer and cut the lath out with the Rotozip, using the wood blade, not the plaster/lath blade.
When we installed the attic hatchway, I went into the attic and found the old wiring which was in the way of the cut. We turned off the power and cut the wire and spliced in new wire and routed it away from the cut. We first attempted making the cut-out in the ceiling using the Rotozip with the plaster/lath blade, but that the blade wasn't deep enough to cut the lath, and it started burning anyway. We resorted to chipping out all the plaster along the cut line and then, while standing on a ladder, cut through the lath using a circular saw. Actually, my wife's uncle did that part. I just held the end of the piece that was being cut out.
The big piece came off in one big dusty crash, and there was a huge dust storm upstairs, which was like fog. Before we could continue, we had to wait for the dust to settle. Then we had to remove part of a floor joist. Then we installed the hatchway.
There wasn't severe damage to the surrounding plaster, and putting door trim around the hatchway hid the plaster cut. All in all, it was a difficult and messy job, but it turned out fine.
If you apply similar principles to how you install the whole-house fan, the surround plaster should be OK. I have thought about installing a whole-house fan in our house, but decide not to after recalling the attic hatchway. firstname.lastname@example.org