Hello Amos and everyone else,
No success, but thought I'd report back that nobody in my local Spanish department knows of the origin of this quote. They recommended looking at Lorca's essays - Nada! (as far as I could see)
It could possibly be one of his very early poems or 'juvenilia' - these aren't included in the standard translated Collected Poems. Or perhaps an interview: these aren't available in collected form.
I'm starting to think this quote qualifies as folklore. It's been passed along, from quote to quote, without any firm evidence of its origins.
I've seen several of 'variants' on the web too. In particular, instead of "cup of beauty", one speaks of "a golden cup"... though perhaps this is evidence it's been differently translated from a Spanish source ????