Thanks to Gonzalo Medina's response and with some modifications I've end up with:
\newcounter{tmp}
\newcommand\tikzmark[1]{%
\tikz[overlay,remember picture] \node (#1) {};}
\def\separacionBarraInicial{.51}
\def\separacionBarraFinal{.53}
\newcommand\Startsquiggly{%
\stepcounter{tmp}%
\tikzmark{a}\label{a\thetmp}%
\ifnum\getpagerefnumber{a\thetmp}=\getpagerefnumber{b\thetmp} \else
\begin{tikzpicture}[overlay, remember picture]
%\draw [decoration={coil,aspect=0},decorate,ultra thick,gray]
\draw [decorate,ultra thick,black]
let \p1 = (a.north), \p2 = (b), \p3 = (current page.center) in
( $ (\x3,\y1) + (\separacionBarraInicial\textwidth,1ex) $ ) --
( $ (\x3,\y1) + (\separacionBarraFinal\textwidth,1ex) $ ) --
( $ (\x3,\y3) + (\separacionBarraFinal\textwidth,-0.51\textheight) $ );
\end{tikzpicture}%
\fi%
}
\newcommand\Endsquiggly{%
\tikzmark{b}\label{b\thetmp}
\ifnum\getpagerefnumber{a\thetmp}=\getpagerefnumber{b\thetmp}
\begin{tikzpicture}[overlay, remember picture]
%\draw [decoration={coil,aspect=0},decorate,ultra thick,gray]
\draw [decorate,ultra thick,black]
let \p1 = (a.north), \p2 = (b), \p3 = (current page.center) in
( $ (\x3,\y1) + (\separacionBarraInicial\textwidth,1ex) $ ) --
( $ (\x3,\y1) + (\separacionBarraFinal\textwidth,1ex) $ ) --
( $ (\x3,\y2) + (\separacionBarraFinal\textwidth,-0.75ex) $ ) --
( $ (\x3,\y2) + (\separacionBarraInicial\textwidth,-0.75ex) $ );
\end{tikzpicture}%
\else
\begin{tikzpicture}[overlay, remember picture]
%\draw [decoration={coil,aspect=0},decorate,ultra thick,gray]
\draw [decorate,ultra thick,black]
let \p1 = (a.north), \p2 = (b), \p3 = (current page.center) in
( $ (\x3,\y3) + (\separacionBarraFinal\textwidth,.495\textheight) $ ) --
( $ (\x3,\y2) + (\separacionBarraFinal\textwidth,-0.75ex) $ ) --
( $ (\x3,\y2) + (\separacionBarraInicial\textwidth,-0.75ex) $ );
\end{tikzpicture}%
\fi
}
\newcommand\Squ[1]{\Startsquiggly#1\Endsquiggly}
I've basically removed the "coil" decoration and make it a bit longer (both in its upper and lower part). And here's the final result

I would like to make a final remark, which is the problems caused by "changebar" to adjust its position (maybe because of a collision with other packages). So the solution with Tikz seems the best one.