Phew, just made a phone call in French to inquire about the times of a bus.
It actually turned out fairly well… I was rather nervous but I understood what the other person said and could even formulate what I wanted (though I had to think at one point because the word that came into my mind was Greek, not French, so I had to reformulate).
I'm not quite convinced we were talking about the same bus line—he said they run every ten to fifteen minutes during the evening, then every twenty minutes until after midnight, whereas the web site (if I understood it correctly) says that the last bus running the whole route is at around 8:30 p.m. and that the bus only serves part of the route (the bit I'm not interested in) after that.
Hm. But at least that means that if I don't try to leave for the hotel until too late, I should be able to catch a bus without having to wait for too long; then I'll see whether there's a timetable posted there. (Or I might try asking at the tourist office, either the branch at Gare du Nord or the main place on Champs-Élysées; I was going to go there anyway.)
(Thanks to angelsk for helping me with my French over AIM.)