accept-line accept-and-hold() {
  if [[ $LASTWIDGET == bracketed-paste ]] && 
     [[ $BUFFER[-1] == $'\n' ]]; then
    zle .backward-delete-char
  fi
  zle .$WIDGET -- "$@"
}
zle -N accept-line
zle -N accept-and-hold

# Maybe do the same for the other accept-* widgets.

(( $+zle_highlight )) || zle_highlight=()
zle_highlight+=( paste:standout )