Use Quick Responses with Google Voice
This is apparently not currently possible.

I'm not the only person with this desire. There's an open issue at Google (Issue 23176).

In one of my searches, I found someone who says they looked into the code for the Phone app, and found that it's calling the Messaging app directly without allowing the Messaging Intent, which is what would prompt me to choose the Messaging app versus Google Voice to send the message.

The app suggested by dymutaos looked somewhat promising, but I am concerned about its security.

So, unfortunately, this very useful feature is currently useless to me. I hope they fix it in the not-too-distant future.