Footnote, non-inline syntax?

The sepfootnotes package might do the trick, but the footnote content has to come before it's usage:

\documentclass{scrartcl}
\usepackage{sepfootnotes}
\newfootnotes{A}
\begin{document}

\Anotecontent{a}{So complex that I would prefer to pull out the footnote text.}

This is a very long complex\Anote{a} sentence.

\end{document}

The following code defines a macro \longfootnote[fn-label]{footnote text}, which produces a footnote and assigns it a label which can be cross-referenced in the usual way using \ref to produce a footnote mark.

\makeatletter
  \def\longfootnote[#1]{%
    \stepcounter{footnote}%
    \@bsphack
      \protected@write\@auxout{}%
        {\string\newlabel{#1}{{%
          \string\begingroup
            \string\c@footnote \number\c@footnote\string\relax
            \string\unrestored@protected@xdef\string\@thefnmark{%
              \string\thefootnote}%
          \string\endgroup
          \string\@footnotemark
        }{\thepage}}}%
    \@esphack
    \protected@xdef\@thefnmark{\thefootnote}
    \@footnotetext}
\makeatother

This code is effectively a composition of of the code from \label and \@xfootnotemark from latex.ltx, writing a label to the auxiliary file which consists of a footnotemark for the new footnote. If you use an editor which recompiles whenever the labels change, you should quickly obtain a new document in which references to the long footnotes appear properly.

Sample document.

\documentclass{article}
\usepackage[left=5mm,right=5mm,paperheight=55mm, paperwidth=62mm]{geometry}
\thispagestyle{empty}
\makeatletter
  \def\longfootnote[#1]{%
    \stepcounter{footnote}%
    \@bsphack
      \protected@write\@auxout{}%
        {\string\newlabel{#1}{{%
          \string\begingroup
            \string\c@footnote \number\c@footnote\string\relax
            \string\unrestored@protected@xdef\string\@thefnmark{%
              \string\thefootnote}%
          \string\endgroup
          \string\@footnotemark
        }{\thepage}}}%
    \@esphack
    \protected@xdef\@thefnmark{\thefootnote}
    \@footnotetext}
\makeatother

\begin{document}
\section{A demonstration}

Here\footnote{foo} is\ref{c} a\ref{a} test\footnote{bar} paragraph.\ref{b}

\longfootnote[a]{baz}
\longfootnote[b]{gleep}
\longfootnote[c]{glorp}
\label{sec:test}

This text\footnote{fie} is a part of Section~\ref{sec:test}.
\end{document}

Result.

The long footnotes should not interfere with any other cross-referencing, as illustrated above.

You might also just define a new command and use it inside the regular \footnote command, e.g.,

\newcommand{\fnOne}{A footnote.}
\newcommand{\fnTwo}{Another footnote.}

A text\footnote{\fnOne} with some\footnote{\fnTwo} footnotes.

Which produces the expected:

One issue with this approach is that you cannot use numbers within command names, which might or might bother you in this case.

That said, I agree that using the setfootnotes package might be the more canonical answer and @Neil de Beaudrop 's answer is more flexible and useful in some scenarios. I'm just providing an alternative.