Random thought: given you already have the gcaa alias, perhaps you could include a check that .git/REBASE_HEAD doesn't exist in that?

Probably easiest as a little shell function like

    gcca() {
      local GIT_DIR
      if ! GIT_DIR=$(git rev-parse --git-dir); then
        return 1
      elif test -f "$GIT_DIR/REBASE_HEAD"; then
        printf 'Rebase in progress: commit --amend is disabled\n' >&2
        return 1
      fi
      git commit -a --amend "$@"
    }

rather than an alias?

[Edit] I forgot about rev-parse --verify, which simplifies this further:

    gcca() {
      if git rev-parse --verify REBASE_HEAD >/dev/null 2>&1; then
        printf 'Rebase in progress: commit --amend is disabled\n' >&2
        return 1
      fi
      git commit -a --amend "$@"
    }

This also leaves you still able to use commit --amend long-hand if (for example) you want to edit one of your own commits during rebase -i.