I am curious if there are other teachers in the building who see this girl's two-faced nature as clearly as you do. Perhaps if a group of teachers were to approach the administrator who approved this not-so-good idea and express their concerns collectively, the assembly could be steered in a safer direction?
Another, sneakier idea that comes into my head is to subtly encourage several other students to ask for their own assemblies immediately. If that starts to happen now, before assembly #1 gets done, maybe those in charge will recognize the problem they have created for themselves and cancel the whole deal.
Giving a drama queen the stage in front of the whole school is a dreadful miscalculation on the part of someone - it sounds like the last thing this student needs is empowerment.