dynamic document root and case insensitive mapping

Hi,
I ve a bigger problem.
We use this config to map subdomain to dynamic document root:

server_name ~^(?<branch>.+?)\.oururl.com$;
root "/var/www/folder-$branch";

It is working, so far, UNTIL we have a non-small-letter in our directory-name. For example, path is:

/var/www/folder-BIG

big.oururl.com is not working because big is not found but BIG exists ...

My first idea was to make a command inside the init-script to make the file-system case-insensitive, but this was not working, either.

Is there any known option like CheckSpelling On/off in Apache to use for it inside Nginx to fix this ? Like mod_speling in Apache?

Symlink / Rename is not possible.

Thanks
Ronny