Is it possible to add the contents of a file to the Nginx proxied request?

Question

Nginx

There is a certain file, let's say my.conf with the content "42". The content of the file is changed by a separate program (it is available for editing, i.e. it can be written to a file in any format).
There is some upstream (compiled, waiting for this number in the Header).
You need to somehow pass 42 to this upstream with nginx.
So far, nginx has not come up with anything worthwhile, except to write
set $var "42"
to the file, include it in the config and reload.