pgfmathdeclarefunction with savebox and tikzexternalize: tikzpicture optimized away

Having solved my previous problem involving savebox and tikzexternalizeI am already facing the next one. I would like to use a function that I have defined either in or after the preamble with pgfmathdeclarefunction in a savebox. It works nicely when using it outside of a tikzpicture, but inside it will get optimized away (see comments in the example below). Why is that so? Or: How can I get TikZ not to optimize the figure away? Note that this solution doesn’t help.

documentclass{article}
usepackage[english]{babel}
usepackage{tikz,pgfplots}
usetikzlibrary{positioning}
usepgfplotslibrary{external}
tikzexternalize

begin{document}
pgfmathdeclarefunction{gauss}{2}{%
    pgfmathparse{1*exp(-(#2)^2/(#1^2))}%
}

newsaveboxboxCPAin
saveboxboxCPAin{
    begin{tikzpicture}
        tikzset{external/optimize=false}%
        draw[domain=-1:1,samples=200] plot(x,{gauss(.33,x)});
    end{tikzpicture}
    }
useboxboxCPAin % works!
begin{tikzpicture}
node (test) {useboxboxCPAin}; % tikzpicture optimized away because it does not contribute to exported PDF
end{tikzpicture}
end{document}