I really think that all technical interview questions should be "open-book". Whether that's doing a homework assignment or simply telling the candidate what technical issues to study up on for the whiteboard.

For the brief stint that I did interviewing, I had candidates whiteboard after doing a homework assignment, but all I had them do was step through the very algorithm they had implemented in the homework using a very simple input dataset. None of them had any issues with the whiteboard, even those who were clearly very nervous/anxious.

Building software is hard enough as it is. I doubt making the coding tasks open book is probably not going to allow in many (if any) unqualified candidates.