Review Board does support post-commit review, but up through the 1.7.x releases, you need to use the command-line tools to create them. See Posting Committed Code for more information. Note that for this usage, the tool is only coincidentally named "rbt post" (or "post-review")--"post" here means "put up", and the command-line tool is used for putting up both pre- and post-commit reviews.
In a future major release, Review Board will be gaining the ability to create post-commit review requests directly from the web UI.