Not considered pure trad, but I think that Donovan and Neil Young played that way.

You can still get a very nice a sound from playing harmonica on a rack, even though you won't get those special tones from using your hands. It does pay to get a good quality rack. If you do happen to have a cheap rack or ( in my case) an old one that's started to 'slip' - you can use gaffer's tape to keep it holding the harp steady. Inelegant solution, but it works.