Bless you, Xposed Framework. Thou art the last refuge of power users whose hardware has been forsaken by manufacturer and ROM developer alike. The root-enabled tool has a new module that back-ports a bunch of Android Nougat features to earlier versions of the OS. It's called AndroidN-ify, and the latest update includes a tweak that exposes Google's fancy new voice-controlled Assistant search tool to users on Android 6.0. An earlier build.prop tweak allowed Android 7.0 users to try Assistant on non-Pixel phones, which won't be officially supported when the new hardware launches.
The update comes from Google+ user Max Rumpf, who sent the adjustment into the open source Github for AndroidN-ify and points users to a copy of the newly-updated APK, here. (Note: this APK module has not been independently verified.) Users will need to be rooted with a working version of the Xposed Framework up and running, and it looks like the module's feature is only applicable to Marshmallow at the moment. Also, the current version of AndroidN-ify on the full Xposed repository says it's intended for AOSP and AOSP-like ROMs, so anyone using a device with a heavily skinned version of Marshmallow probably shouldn't try it. As with all Xposed modules, a full backup before enabling it probably isn't a bad idea.
Google has courted controversy by making the fully OS-integrated version of Assistant an exclusive to the new Pixel phones, at least initially. Assistant is also available as part of the Allo chat app, and will come in stand-alone hardware form with Google Home. Nexus phones may or may not get stock access to Assistant with the upcoming Android 7.1 developer preview.