This is not really a woodworking question, but I figure someone here
can answer it.

My Dad is 92 and needs to grab onto things to climb the stairs. But
for a short section of the stairway in their house, there's no
sensible thing to grab.

Their house is plaster on lath if memory serves. Wood strips, I think,
not wire. [This is an ancient memory. My parents had a larger closet
made when I was a very young boy and I was allowed to bash away at the
plaster for a little while. I remember being disappointed when I came
back from school and found that the rest of the demolition had already
been completed. ]

Anyway, I have no experience finding studs in that kind of
construction. The walls are too rigid for me to just sound them out
with my fist. I don't own a stud finder, but I might buy one if it's
of any use in that kind of wall. I'm not eager to drill a series of
holes that I'll need to repair, and then match the paint. If it comes
down to it, I suppose I could remove the base molding and drill holes
at the bottom of the wall, but the molding is finished oak, and a
little complicated, and would be very hard to replace if an oaf like
me were to damage it.

Suggestions? I imagine I'd use oak railing parts from the Borg, by the
way.