I've seen some photos recently of their meeting. Does anyone know when this was, and was it for a particular cause?

It was during his New Power Soul days just after Emancipation.
It was for particular cause.

Prince was interviewed about it during a press conference, and they asked him how he got involved. He said that he was given a call, and that he was so eager to assist in the cause that he offered to even "mow the lawn."

I am looking forward to that gig that they were discussing. I'll put that one on the list of things Prince discussed and didn't deliver on. That will go up on my shelf right next to the 7 disk sample set.