Move shaded polygon behind chemfig schemes

You could put it on a background layer, but why bother? Does it matter if it is behind? Or only that it looks as if it is behind? If the latter, what about the following?

looks behind?

\documentclass{article}
\usepackage{chemfig}

\begin{document}
%\schemedebug{true} % uncomment to see node names
\schemestart
P \arrow(P--Q){<=>[$\mathrm{k_{pq}}$]} Q
\arrow{<=>[$\mathrm{k_{qs}}$]}[30] S \arrow(S--T){<=>[][$\mathrm{k_{st}}$]}[-30] T
\arrow(@Q--R){<=>[][$\mathrm{k_{qr}}$]}[-30] R
\arrow(@[email protected]){<=>[][$\mathrm{k_{rt}}$]}
\arrow(@T--U){<=>[][$\mathrm{k_{tu}}$]} U
\arrow(@U--V){->[$\mathrm{k_{uv}}$]} V
%
\chemmove{%
  \draw [red,fill=red,fill opacity=0.5,blend mode=multiply,thick] ([shift={(-5pt,1em)}]Q.north west) -- ([yshift=1em]S.north) --
                    ([yshift=1em]T.north) -- ([shift={(5pt,1em)}]U.north east) --
                    ([shift={(5pt,-1.5em)}]U.south east) --
                    ([yshift=-1.5em]T.south) -- ([yshift=-2em]R.south) --
                    ([shift={(-5pt,-2em)}]Q.south west) -- cycle;
}
\schemestop
\end{document}

Just in case you really want it behind, there is always the eso-pic rescue.

\documentclass{article}
\usepackage{chemfig}
\usepackage{eso-pic}

\begin{document}
%\schemedebug{true} % uncomment to see node names
\schemestart
%
P \arrow(P--Q){<=>[$\mathrm{k_{pq}}$]} Q
\arrow{<=>[$\mathrm{k_{qs}}$]}[30] S \arrow(S--T){<=>[][$\mathrm{k_{st}}$]}[-30] T
\arrow(@Q--R){<=>[][$\mathrm{k_{qr}}$]}[-30] R
\arrow(@[email protected]){<=>[][$\mathrm{k_{rt}}$]}
\arrow(@T--U){<=>[][$\mathrm{k_{tu}}$]} U
\arrow(@U--V){->[$\mathrm{k_{uv}}$]} V
%
% \chemmove{%
% }
\schemestop
\AddToShipoutPictureBG*{
\tikz[overlay,remember picture]{
  \draw [red,fill=red,opacity=0.5,thick,semitransparent] ([shift={(-5pt,1em)}]Q.north west) -- ([yshift=1em]S.north) --
                    ([yshift=1em]T.north) -- ([shift={(5pt,1em)}]U.north east) --
                    ([shift={(5pt,-1.5em)}]U.south east) --
                    ([yshift=-1.5em]T.south) -- ([yshift=-2em]R.south) --
                    ([shift={(-5pt,-2em)}]Q.south west) -- cycle;
}
}
\end{document}

enter image description here

For the example at hand, @cfr's solution is probably simpler, but if the foreground has colors, this method might be considered.