I dined once, in February, and the Imagineer was Deb Peterson, a graphics design engineer who worked on many high profile attractions. The last two years being spent on Expedition Everest and Hong kong Disney. Among other things, she was responsible for the signage of the attractions. Who you will get, depends on the work schedule of the imagineers, but, she suggested that you would be able to find out who would be dining on a particular day so as not to get repeat imagineers if you have done it before. Its a great experience, well worth doing again. You cant book it any further out than 30 days, my advice is to book it early on day 30. seating is limited to about ten. Also, the AKL has recently started a similar dining experience with the animal keepers.
Hope this helps,
Lancer