Opera Next was added because avast! BETA was being tested during that time and it was good time to test it. Otherwise, Opera Next is like a very early beta, nightly snapshots basically. You can't expect them to keep track and update plugins for a nightly versions which can break things between releases.
You'll only see plugins for final versions.
For Opera Next, you can install WebRep manually by goint to the avast! folder:
C:\Program Files\AVAST Software\Avast\WebRep\Opera
Drag file wrc.oex into the Opera window and it will ask you to install the plugin.
Thats how i used it in Opera Next versions and it worked perfectly fine.