Currently I'm reading "Facing The Other Way - The Story Of 4AD" bij Martin Aston. It appears that while preparing the second album of the project This Mortal Coil producer and label-owner Ivo Watts-Russell had asked three iconic singers to perform on it: Scott Walker, Robert Wyatt and ... David Sylvian! Wyatt was the only one who gave a response ("No"), but it's interesting to fantasize on which track of "Filigree & Shadow" David would have lent his voice. One of my favorite choices would have been "The Jeweller", originally from Pearls Before Swine.
Here's a funny fragment from the book:
"One Poker player who hung around the studio was [Jon] Turner's friend Les McKeown, the former singer of the Scottish Seventies pop stars The Bay City Rollers, an obsession of many a pre-pubescent and teenage girl. "Late at night", Ivo recalls, "I'd often play a cassette with David Sylvian's "Brilliant Trees" on one side and Scott Walker's "Climate Of Hunter" on the other. Les would say: "Who's this, then? Do people make a living out of this kind of music?""