Hi David,
I was thinking the wrong way : moodle is not a CMS and is not designed to catch one specific page link by saying "fetch me the URL of the page which name is xxxx (or id is X)".
Instead, I will create a theme settings admin page where the user will past the url of the page he wants to link to. It should be enough for my needs right now.
Your point "always use moodle_url() to generate urls" - I'll keep that in mind.
Thank you