Depending on your roof pitch, crawling in that close to the eaves might be tight. Try cutting a small hole in the lathe and plaster in between the studs, as close to the top of the wall without damaging the trim and blow in the insulation. ( like blowing insulation in the attic ) Depending on how close to the ceiling you were able to cut the hole, you might have to stuff some fibreglass insulation just to fill in the gap to the top plate of the wall. Then just patch the small hole used.

Without destroying all the lathe and plaster and possibly damaging the trim by removing it, I can't see what else you can do.