Hello James,
Unfortunately it is my understanding that this feature is disabled when in sequence mode due to the timing challenges it creates when trying add additional scenes of different text lengths. Since each take item is a separate scene setting a duration via sequence would result in XPression trying to distribute the time evenly between each scene. This will not work because each scene could have paragraphs of drastically different lengths resulting in a roll that speeds up and slows down.
So long story short nothing you are doing wrong, and not a bug but probably something we at Ross should investigate implementing a bit better.
As an alternative have you considered using a scene group in layout but driving it with an external data source to make it easier to update? A simple text file connected via Datalinq would do the trick, that is assuming you have the Datalinq option. Otherwise you will have to type it out in layout for the time being
Regards
Andrew
#XPression