Not exactly a 'visual gag', but in "The Curse of Fenric" the Doctor (Sylvester McCoy) and Ace (Sophie Aldred) travel back to wartime England. Whilst the Doctor keeps his customary garb, Ace dresses in period clothes, complete with stockings and suspenders.
Of course, you need to be really sad, and use 'freeze-frame' to spot the one shot of stocking tops that got released on the video!

Nigel