I've never grown one in an upside-down pot myself. However, Goulding's "Fuchsias: The Complete Guide" (New ed.) has directions for growing fuchsias in a hanging bottle: the plant hangs out the bottom end of the bottle (which is, in turn, hanging from the neck). I therefore predict that if it works for a bottle, it will work for an upside-down pot.

By the way, according to Goulding, "Small-flowered types like 'La Campanella' or 'Zulu Qeen' tend to do best when planted in such oddities" like bottles (p. 62).