Thanks for the responses! On the S80 I was on, the button was on the side of the arm rest (not the under-side) back where it attaches to the seat. If while seated you stick your thumb in the corner of your left back pocket at wave your fingers toward the armrest, you'll find it.

This was in a D seat. The poster above may have been right about only the D side having liftable aisle armrests.

My knee thanks you, my other knee thanks you, and I thank you.