I got this idea from a book called 'Flash training' by Eric horst. It is basically a book for rock climbers, but has some good training ideas on pull ups. Anyway whenever the WOD requires pull-ups/dips etc for a period of time I always use a workout that is included in this book. Basically you start your stopwatch off and do five pull ups and then rest for the reminder of the minute, when the second minute starts you do another 5 pull ups and rest the reminder of the minute and so on for 20 min. For the WOD I modified it by adding 5 dips straight after the pull ups and carryed on for the 30 mins. It certainatly was'nt easy after 20 min.
Hope this helps some people Thanks matt Cullen.