Unfortunately, no Droid app can be started with the BT "voice dial" command button at this time. The OS has disabled this capability. (Specifically, this refers to the "voice recognition activation" feature that is an optional part of the Bluetooth Hands-Free Profile specification.)
You can still operate without touching the phone's screen and call through your nuvi if you make a couple compromises. First, you will press the camera button to start. Second, the phone needs to be out so it can use the speakerphone/mic to have the "which number" conversation. After that, the call will go through your nuvi.
Frankly, I realize this is not a great solution for you--with a nuvi you are unlikely to want to also dock your Droid, or even have it out. We expect Google to remedy this at some point and intend to enable BT initiation when able. (Or perhaps at the same time Google will release their own hands-free voice dialer, and users can save the $4.)
As an interesting aside, some BT devices use their own voice dialer (and do not rely on the phone). These units download your address book from the phone, do the speech recognition themselves, then use BT to send a "call this number" command to the phone. This is supported today in the Droid. It has led to some "my BT voice dial button works, why doesn't yours" confusion.