>> In a Rolling Stone interview, Mick said that it was Brian playing slide on that song. <<
maybe this is the quote you mean - from [
www.timeisonourside.com]
That's Brian playing (steel guitar). We were sitting around in a circle on the floor, singing and playing, recording with open mikes.
That was the last time I remember Brian really being totally involved in something that was really worth doing. He was there with everyone else.
It's funny how you remember - but that was the last moment I remember him doing that, because he had just lost interest in everything.
- Mick Jagger, 1995