Hi Richard and all,
Yes, I use an armrest on my Vessel mandolin. It’s held on the instrument with the same clamp that they use on violin chin rests. But that won’t work for guitar. They don’t make big enough clamps!
I used a black plastic John Pearse on my Guild G-37 before I sold that to Chad Manning.
The adhesive strip in the armrest did zero damage to the finish, though I have no idea what Guild was using for finishes in 1975.
Hope that helps!