I finally got around to installing a turn signal beeper on Wife's motorcycle. Rather than a simple beeper that's on whenever the signal is on. I did one that's on only when the signal is on...and the brakes are off. With brakes on, the buzzer shuts up. Much easier than I expected. The trick is to ground the buzzer through the positive wire of the brake light. When the brakes are off, there is a path to ground through the bulb--you don't get full voltage, but plenty to run the buzzer whenever the turn signal is on. When the brakes are on, you get 12 volts on both sides of the buzzer...so no sound.
An additional benefit is that a blown out brake light bulb will keep the buzzer from working, and a bad brake light switch (a problem Wife's Savage had when we got it) means that the buzzer will be on even with the brakes applied.