We have done the segway tour and loved it. We were able to book our tour a year before we did it so you should be able to phone and book now. I don't think you will be able to watch most of the tour though. The 1st hour is inside the training centre and then the 2nd hour around the world showcase which will not be open to the public at that point. You will be able to see your DH when they are driving from the training room to the world showcase.
Have fun, it is a great tour one I will definately do again.