@sdetweil Ok, implementing @sdetweil 's suggestions, I have created a working version. This is my workflow.

// config/dictionary.txt
// This is a list of key-value or string-replacement pairs
"{latitude}"    :    "40"
"{longitude}"    :    "-70"

// installers/copy-values.sh
// sed command found here: 
// https://unix.stackexchange.com/questions/269368/string-replacement-using-a-dictionary
cd ~/MagicMirror
cp config/config.model.js config/config.js
sed '
s|"\(.*\)"[[:blank:]]*:[[:blank:]]*"\(.*\)"|\1\
\2|
h
s|.*\n||
s|[\&/]|\\&|g
x
s|\n.*||
s|[[\.*^$/]|\\&|g
G
s|\(.*\)\n\(.*\)|s/\1/\2/g|
' config/dictionary.txt | sed -i -f - config/config.js

DISPLAY=:0 npm start

Now I can have separate dictionary.txt files on both machines for any local important information, which I will add to .gitignore. I will continue to modify config.model.js, and config.model.js will be copied and cleaned up with all the right values before every start of the mirror.