It just works* (tm)

* move user up one level, change config as documented.
* if installing on subfolder, run this on root forum directory; grep -rl theme | xargs sed -i 's=\/themes=/forum/themes=g'

Working with Nginx under subfolder;

git remote add upstream https://github.com/Kroc/NoNonsenseForum.git
git fetch --all
git checkout -b mirror-v26 v26
git checkout master
git checkout develop
grep -rl theme | xargs sed -i 's=\/themes=/forum/themes=g'
git add -u .
git commit -m "fix theme base url, moved from root to forum subfolder"
grep -rl thread --exclude-dir=.git | xargs sed -i 's=thread\.php=forum/thread.php=g'
git add -u .
git commit -m "fix thread.php base url, moved from root to forum subfolder"
grep -rl index --exclude-dir=.git | xargs sed -i 's=index\.php=forum/index.php=g'
git add -u .
git commit -m "fix index.php base url, moved from root to forum subfolder"
git push

After few lines of code discover FORUM_PATH, but not yet discovered where is assigned. Above solution works, just need to fix logo and home link.

It's working on my server, this solution is ART !!!