I recall that Robert Fripp of King Crimson has posted to the KC website and answered questions posed to him.
There’s a fansite for Quebec legend Michel Pagliaro. I don’t believe he’s posted to it yet, but he knows it exists and has met several of the people who run the site and answered their questions about his old recordings and sessions.
I heard a record on the Dr. Demento Show back in the '70s, called “Friendly Neighborhood Narco Agent”, by Jef Jaisun. I had been looking for it for more than 20 years, when I went looking for it online, and found Jef’s website! I asked about the record, and after trading a few e-mails, he sold me an original pressing of it.
The only other experience I have had directly is a response from Eric Carmen (Raspberries) to a question I posted through his webmaster (the middleman).