Q:

How to set koma pseudolength in emacs org mode?

Using emacs org-mode and the koma letter script, how can I set the vertical address position?
The 1manual at p.484 displays a nice 2overview of position possibilities.
However, neither of these lines result as expected:
#+LATEX_HEADER: \setplength{toaddrvpos}{5cm}
#+LATEX_HEADER: \@setplength{toaddrvpos}{5cm}
Example.org
#+LATEX_CLASS: default-koma-letter
#+LATEX_HEADER: \setkomavar{signature}{\includegraphics[height=1cm]{signature.png}\\\usekomavar{fromname}}
#+LCO: SN
#+LANGUAGE: en
#+LATEX_HEADER: \setplength{toaddrvpos}{5cm}

#+AUTHOR: Bugs Bunny
#+TO_ADDRESS: Batman
#+SUBJECT: My subject
#+OPENING: My opening
#+CLOSING: My closing
#+FROM: From me

Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Donec hendrerit tempor
tellus. Donec pretium posuere tellus. Proin quam nisl, tincidunt et, mattis
eget, convallis nec, purus. Cum sociis natoque penatibus et magnis dis
parturient montes, nascetur ridiculus mus. Nulla posuere. Donec vitae dolor.
Nullam tristique diam non turpis. Cras placerat accumsan nulla. Nullam rutrum.
Nam vestibulum accumsan nisl.

Footnotes
1 http://texdoc.net/texmf-dist/doc/latex/koma-script/scrguien.pdf
2 

A:

Define an own lco file (recommended solution):
\ProvidesFile{mySN.lco}[2019/04/13 letter-class-option]
\LoadLetterOption{SN}
\let\raggedsignature=\raggedright
\setplength{toaddrvpos}{10cm}
\endinput

Then
\documentclass[foldmarks,backaddress=false]{scrlttr2}
\usepackage{lipsum}% only for dummy text

\LoadLetterOption{visualize}% can be used only in preamble
\showfields{address}% visualize the position of the address field

\usepackage{graphicx}
\LoadLetterOption{mySN}% load the user defined lco file
\setkomavar{fromname}{Bugs Bunny}
\setkomavar{subject}{My Subject}
\setkomavar{signature}{\includegraphics[height=1cm]{example-image}\\\usekomavar{fromname}}

\begin{document}
\begin{letter}{Batman}
\opening{My opening}
\lipsum[1]
\closing{My closing}
\end{letter}
\end{document}

results in 

With older KOMA-Script versions (3.25 or earlier) you have to replace \setplength by \@setplength in the lco file.

With \AtBeginLetter and SN.lco:
\documentclass[foldmarks,backaddress=false]{scrlttr2}
\usepackage{lipsum}% only for dummy text

\LoadLetterOption{visualize}% can be used only in preamble
\showfields{address}% visualize the position of the address field

\usepackage{graphicx}
\AtBeginLetter{%
  \renewcommand{\raggedsignature}{\raggedright}%
  \setplength{toaddrvpos}{10cm}%
}
\LoadLetterOption{SN}
\setkomavar{fromname}{Bugs Bunny}
\setkomavar{subject}{My Subject}
\setkomavar{signature}{\includegraphics[height=1cm]{example-image}\\\usekomavar{fromname}}

\begin{document}
\begin{letter}{Batman}
\opening{My opening}
\lipsum[1]
\closing{My closing}
\end{letter}
\end{document}

The result is the same as above.
Note: With older KOMA-Script versions (3.25 or earlier) and this solution you have to replace 
\setplength{toaddrvpos}{10cm}%

by
\makeatletter\@setplength{toaddrvpos}{10cm}\makeatother

because \AtBeginLetter is used in the preamble of the document.

