Yes, Utilisateur: and Utilisatrice: will work. The software looks for links to user pages; you can think of it as doing a reverse Special:Whatlinkshere / Spécial:Pages liées on the mentor list page. If the mentor list shows up in the Whatlinkshere list for your user page, then you are a mentor.
The placeholder for users without a description is "Cet utilisateur expérimenté sait que vous êtes nouveau et peut vous aider dans l’édition." (translated here, can be customized here)
Transcluded lists will work partially, but not fully. Links to user pages in transcluded lists will be recognized, but descriptions will not be. So these users will be mentors, but their descriptions will not be displayed.
However, for absent and inactive users you may not want this to work: presumably absent/inactive mentors shouldn't be assigned new mentees? In that case it would be better to list their names as plain text, not as links, so that the software doesn't recognize them as mentors. Removing a mentor from the list (or removing the link to their user page, at least) means they don't get assigned new mentees, but their existing mentees don't get reassigned to new mentors.