Can I configure voice dialer to be usable in hands-free mode?
Yeah bro there is an app on Google Play Store that can control your headset button actions, The app will help you customize the options when a button is pressed on your device, so with that app you can go to "all apps" and select the name of the voice dialer program, so next time you click on the heaset button, the mobile does work for u

link for app
https://play.google.com/store/apps/details?id=tvk.headvol 

if u find it helpful u can accept my answer