Sounds like you may want to try the native Voice Assistant project. For Google/Alexa these are pretty standard and not able to be changed. Users can change on an account-level to ‘Hey Echo’ but this won’t affect your creating experience and what you’re expecting.
speaking about alexa/google, wake word is for activating microphone of smart speakers and its feature is built-in inside of smart speakers, which is not controlled by Voiceflow, IMO. after microphone is activated, we trigger apps by saying invocation with app name, which Voiceflow can set via Amazon/Google API. this combination of wake word and invocation is defined by alexa/google platform.
for custom assistant with your own wake word, that means you will built your own custom smart speakers and implement wake word feature inside, I guess. is this what you expect?