All Postfix configurations with file expansion

Solution 1:

where can you find all Postfix configurations accepting a file in Debian?

There's just that one. (It's also a Debian-specific addition – "upstream" Postfix actually doesn't support reading the mailname from a file, it's just a regular string.)

Since I'm trying to keep the configuration as modular as possible using dedicated files (e.g. easy to be updated by administration scripts)

I'd say that's the wrong approach – your administration scripts shouldn't be trying to edit individual lines directly. If you want to change a single setting, Postfix comes with tools for doing this; just run postconf myorigin="new value" from the script and it will update main.cf using the correct syntax.

(Though if you're managing many servers, it might be better to use configuration deployment tools to generate the entire main.cf from a template.)