The main problem would lie in getting permission from all the contributors. As rights were not assigned, apart from online ones, permissions would have to be gained. While many of the contributors continue to post here and could easily be contacted in order for them to assign the rights, I bet there are some who have disappeared from the board.
However.
When I worked as an editor, all those years ago, I worked on a book which contained a whole bunch of quotes, many of which were impossible to get permission for: although all the sources were, mostly, attributed, some of the quotes came from magazines long out of print, while others came from books which had been self-published by people who were impossible to track down.
As the book was written by my publisher, I could not insist that he get the permissions himself. It was My Job.
I duly contacted everyone I could, and got the permissions that were gettable. All the others I tried to track down, but in most cases, I failed.
Consequently, I included an acknowledgements page in which I thanked all the rights-holders who had granted permission for the quotes to be used; and a para which read something like this: while every effort had been made to gain permission for all quotes used, in some cases, that had proved impossible; but that everything possible would be done to get those permissions for all future editions. And that if any permission-holders wanted to contact the publishers they could do so at 123 What Ever Road It Was.
No one ever contacted the publishers, and all was well.
If I still have a copy of the book concerned I'd be happy to give the exact wording. It was checked by the company's lawyers, and by several others too. And if I can't find it, I bet several of the writers here could provide an appropriate wording.
