I wouldn't have thought that such a small a bit of modern plaster / filler is going to significantly impair the permeability of the wall as a whole (particularly as most modern paints are going to be doing that anyway).
There are some bits where the plaster has blown and pulled away when we’ve removed wallpaper - would you still think that bonding and easifil be the way to go?
We’re going to try and use clay paint where budget permits as well.
Bonding + finish plaster or easifill is the standard approach, it's pretty easy to get it done quickly and unlikely to create too many problems. Sounds like you might not be happy unless you do it in lime though.
I wouldn't have thought that such a small a bit of modern plaster / filler is going to significantly impair the permeability of the wall as a whole (particularly as most modern paints are going to be doing that anyway).