If you're adding fix-up commits rather than amending, you'd want to post the range of commits for the whole branch corresponding to the change. This can use git's revision syntax, for example, if you branched off master:
rbt post -r XX master..HEAD
By default, rbt post will post all changes between the configured TRACKING_BRANCH and your current HEAD, so if that's set correctly for your current branch (either in .reviewboardrc or the default of "origin/master"), "rbt post -u" should still do the right thing.
David