It looks like most of what I would have replied has been replied already aside from one thing...
It is just NEVER EVER appropriate that the SM be reading or working on anything non-show related in rehearsal. If I ever had an assistant be reading or crossword puzzling etc... I'd be mortified and to be perfectly frank; they'd probably never work for me again.
Music rehearsal is a great chance for you to give your assistant's the night off, or a chance for them to be working on paperwork etc... If you have already done absolutely everything you can think of to do in connection to the show, and it's the 76849th hour of music rehearsal for the same 3 bar's of music, then you could ask the music director if they would mind if you worked in your office/ computer lab for the next 20 minutes or so. But I would be worried about being out of the rehearsal room for more than 20 minutes- you'd be bound to miss something.
I'd really suggest that you spend the time with the score in music rehearsal learning every nuance of every note to be played- it will really help you out when it comes to calling the show.
Just my suggestions.... Enjoy your musical!