To follow up on what Sveta said, I don't think it's worth the trouble unless you are already rooted. I'm not opposed to rooting, and actually have rooted most of my Android phones and tablets over the years. You do lose the automatic updates to Android if you root your Nexus 6, I'm pretty sure. It's not a huge deal, since you can install the updates yourself, but it's something to consider.
I also have to wonder if this will be something that the "M" update will activate. Maybe not, but there is a chance. I don't see any reason why Google would nerf tap to wake since the capability is already built into the Nexus 6.