Yes, I would call him Jack, just as I refer to Mr. Doak as Tom (no pun intended) on this website.
On a website, I see everybody as equal. Everyone calls everyone else by their first name and it should be like that, online.
When I was at Tumble Creek I tried my best to always say Mr. Doak, out of respect and relative awe of being in his presence. The way I see it, it's generally is a lot more respectful to call someone Mr. ____ or Mrs. ____, especially upon first introduction. I learned that as I began working at Seattle CC. Always say Mr./Mrs./Dr. . So, when I first met Tom Doak, I called him Mr. Doak, as think is the right thing to do. I see no problem with calling him Tom online. And, as far as I can tell, he has no problem with people referring to him as Tom, just as others refer to fellow gca'ers by there first name. Just an observation.
But yes, I would call him Jack online, if was indeed a poster.
If I ever were to actually meet him, it would for sure be Mr. Nicklaus though.