One word: don’t!
- you’ll slip off the pedals
- or, you’ll waste concentration and energy making sure you don’t slip
- you’ll stretch the back of your heel too far
- playing a third will be impossible (C and E, say, with toe and heel at the same time ~ your instep will play the D in between the notes you want ~ you need a shoe with a heel)
Organ shoes rock! If you use them only to play the organ (i.e. don’t even wear them outside), they will last for years. You’ll be glad you took the plunge.