The best method is what you described, connection to the in-room PA gear.
That also assumes that:
A. you do have permission
B. the DJ or other operator can handle the live mix correctly
C. the in-room gear works properly
D. you have the right adapters and proper level inputs on your gear
Another tried-and-true solution is to gaffer-tape your own small mike "onto" the in-room PA mic used by the speech-makers.
YOU would have control of the mic and it would be inches from the mouths of those speaking.