Previously, rbt post with the submit-as option could only be used for creating review requests, not for updating them. The function guess_existing_review_request_id searched for review requests of the logged-in user, not the user specified by submit-as.
In order to allow updating of review requests that are not yet public, I've proposed a few changes in reviewboard (see #7196). This allows users with the can_submit_as_another_user permission to list nonpublic review requests.

Publishing with rbt post --submit-as=submitter, then updating the review request with rbt post -u --submit-as=submitter both before and after it has been made public
Running unit tests