I did something very similar with a bash function:

  function git-commit-fixup() {
    git commit --fixup ":/$*"
  }
  alias gcf="git-commit-fixup"
  # Looks for the most recent commit that matches its arg
  # eg: we have three commits with messages: 
  #   "fix: the thing" 
  #   "feat: 5 percent cooler"
  #   "test: test coolness"
  # then we do some work and git add, then do:
  gcf cooler"
  # now we have 4 commits: "
  #   "fix: the thing" 
  #   "feat: 5 percent cooler"
  #   "test: test coolness"
  #   "fixup! feat: 5 percent cooler"
  # And autosquash will combine the fixup commit with the appropriate semantic commit as you say.

Sadly I've stopped using it as someone introduced a bunch of commit lint git hooks that choke horribly on the "fixup!" part. And you can't pass --no-verify while rebasing.