I've been to a couple dan tests where all the yudansha present were asked to sit around the mat during the test and to join the teachers (two or three of them) who formed the actual testing panel for discussion afterwards.
When my Sensei conducts testing all yudansha are asked to grade the test. We are not part of the testing board but what we do is compare how we graded the test with how the people on the board graded the test. We get to ask them questions and determine why they scored it as they did. This is all in an effort to teach us what to look for and how to grade tests.