My call for help with the botanical name of the cucuzza growing on the roof was heeded by two kind Capetonian botanists. Thank you! Donovan Kirkwood confirmed its status as a gourd and sent me a scan of a snake gourd, as well as a couple of excellent vegetable reference sources. Which led me, with my tail tucked between my legs, to two of my own books, which it had not occurred to me to consult (is that English?).
In Elizabeth Schneider's beautiful and useful Vegetables from Amaranth to Zucchini (which I purchased after failing to identify too many vegetables and leaves in Chinatown), I found it, after three dips into the index.
Lagenaria sicecaria "Longissima" - cucuzza.
Schneider confirms the use of the youngest leaves and shoots as a cooked green; I'll pick some later and include them in saute of lamb's quarters (fat hen) and pigweed which I picked yesterday (from the terarce and roof). She says they are slightly bitter, but I like bitter. It's the smell I'm worried about. Working up there I was convinced that my hair was on fire.
I'm not sure that the gourds will grow very mature, lying down on the warm roof. I think we'll eat them young, and I'll leave some to see whether they get big.
It's still grooooooowiiiiiing.....