[Note: This is something that I've known about for many years, having a copy of a copy of the message forwarded to me by a third party. However, recently I discovered details of the source of the information, including an approximate date for when Martin provided his response.]
<< Who is the couple celebrating the birth of a son that Dany sees in her vision in the wizard's palace in Qarth? Can you tell us? Is it Rhaegar and someone? Or is it the original Aegon (the Conqueror?) >>
Rhaegar and his wife, Elia of Dorne.