Is the "Secret Policeman's Ball" an allusion to bribery?

Yes. Although you might be confusing two different uses.

The Secret Policeman's Ball was an Amnesty International show, like a Broadway show.

It got its name from a running joke in British culture at the time that said if you got pulled over by the police, you could offer to buy a couple of tickets to the Policeman's Ball and bribe your way out of the infraction.

See this article for more information.