To expand on what John said, the point of a boundary mic is that it must be on a hard, flat surface. By being in that position, it picks up only the direct sound waves, not any reflections off said hard, flat surface. (The physics is more complex but that'll do for setting up).
Boundary mics are most commonly used in theatres (along the front edge of the stage) to provide general sound reinforcement or on tables at conferences for basic recording of what's being said. However, I've seen/heard decent results from boundary mics placed on walls and even ceilings. This is the reason John was saying you'll have to experiment with the location.
One other thought though--your post at least implies that the event is being miked up for sound reinforcement purposes. I know you said the PA and broadcast systems are separate, but don't discount getting splitter cables or boxes and feeding the same sources into both systems. Or, another option would be to use an aux on the PA mixer to create a separate mix specifically for the simulcast. Depending on the size of the room and how elaborate the event is, either of those might produce better results than a single boundary mic.
