Thank you everyone for taking time out to help. I have access to one
wall from a crawl space, but from there I'm on my own. I think running
the NM cable sideways will be cleaner and keep the plaster damage down
to a minimum.

Notching the stud and getting the nail plate in would be impossible
without severing the lathe straps, which are mounted directly onto the
stud. The studs are also sideways back-to-back, not like newer
construction, so I'd have to drill through 4" of elderly timber.
Couldn't I just sneak the cable in between the straps of lathe and
cover that with plates? That does sound remarkably easy, though. Too
easy.

Thanks - dan