Kipling uses the word that way in “A Friend’s Friend”, Plain Tales from the Hills, 1888. The fictional Kipling takes his guest Jevon to a ball, and Jevon gets hopelessly drunk, annoys everybody, and is embarrassing Kipling. Then Kipling goes on:
I set him [Jevon] in a quiet corner of the supper-room, and went to find a wall-prop that I could trust. There was a good and kindly Subaltern ― may Heaven bless that Subaltern, and make him a Commander-in-Chief! ― who heard of my trouble. He was not dancing himself, and he owned a head like five-year-old teak-baulks. He said that he would look after Jevon till the end of the ball.
The Kipling Society confirms that a wall-prop is a non-dancer who leans against a wall.
Now, I haven’t been able to find the word used that way anywhere else. So did Kipling coin it himself and it didn’t catch on, or was it used with that meaning somewhere?