If his explanation about the nature of the patronus was believed, the active patronuses should dispel.
So if he demonstrated and convincingly explained his patronus and chose not to destroy the dementor he would be the sole person in the room with control over the creature.
EDIT:
The trouble is, that even if he manages to control the Wizengamot in this way, he can only control them for so long, as they remain in the room.
So he probably would have to rely on their status and/or ‘Most Ancient Tradition’ keeping them from fleeing the room or calling in reinforcements, while he threatens to release the dementor and/or the information.
(Note that he wouldn’t have to reveal his dementor-destroying ability this way, just his ‘perfect shield’.)
That would antagonize the wizengamot very, very badly. Not a good long term strategy, even if you intend to dissolve it. Destroying it, explaining, and letting them work through the logical implications on their own suffices. If most people lose the ability to halt dementors at all, and a few people gain the ability to destroy them outright, they are no longer usable tools of enforcement, and disposing of them all together becomes quite urgent.
If his explanation about the nature of the patronus was believed, the active patronuses should dispel. So if he demonstrated and convincingly explained his patronus and chose not to destroy the dementor he would be the sole person in the room with control over the creature.
EDIT: The trouble is, that even if he manages to control the Wizengamot in this way, he can only control them for so long, as they remain in the room.
So he probably would have to rely on their status and/or ‘Most Ancient Tradition’ keeping them from fleeing the room or calling in reinforcements, while he threatens to release the dementor and/or the information.
(Note that he wouldn’t have to reveal his dementor-destroying ability this way, just his ‘perfect shield’.)
That would antagonize the wizengamot very, very badly. Not a good long term strategy, even if you intend to dissolve it. Destroying it, explaining, and letting them work through the logical implications on their own suffices. If most people lose the ability to halt dementors at all, and a few people gain the ability to destroy them outright, they are no longer usable tools of enforcement, and disposing of them all together becomes quite urgent.