As soon as Gerrit knows about a change on any branch, it can't be pushed as a new change to another, reviewed branch.
Example:
push 3 changes to "tmp/mybranch"
push the same 3 changes to "refs/for/master"
Gerrit will say "No new changes"
The %base feature can be used to make new changes out of this. But still the review can't be enforced for this branch. A user could "squash" the last 2 commits and with a push to refs/for/master also the first commit comes into master (after the squashed commit is submitted).
To offer a kind of playground or backup branch, it would be good to offer an option (per project) to exclude certain branches from the change tracking, e.g. "tmp/*"