Have been asked a few times but never actually had anyone wank into my wife's knickers/panties.
Would be hot though, especially wrapped around a younger, big, thick cock and the guy was looking at pictures of my wife turning him on even more so he shoots a big load into them!
Love the feel of a ladies knickers wrapped around my cock. Best experience was having a couple invite me to cum into the wives knickers, outdoors, as she held them out and he took pictures. Was AMAZING!