A piano in accommodation is likely to be set up to entertain the guests, and practicing may annoy the other guests. It also might not be very well tuned. So you might prefer to look for accommodation in a city and rent a music studio by the hour.
Unfortunately I can't offer a simple way to find one. (But it's not a field I know anything about, maybe a French musician would know more.) On the web, I can find some sites that list music studios but they don't list many of them, they look like they might be out of date, and they often don't have much information other than contact details. Some sites have an English interface but beware that it may list less information than in French. So Google is probably your best bet.
A piano in French is “piano” (“piano à queue” for a grand piano, “piano droit” for an upright piano). A place where you can play and record music is a “studio de musique” or “studio” for short (but note that “studio” also means a small apartment!). A more precise name would be “studio de répétition” or “salle de répétition” (a room for musical practice, as opposed to “studio d'enregistrement” which has recording equipment and is presumably a lot more expensive), but this term isn't used everywhere. You want one “avec piano” or “avec un piano” (with [a] piano) or any other term that indicates that there is a piano. You rent by the hour (“à l'heure” or “par heure”).
Another word to look for is “médiathèque” (public media library). Some have a piano room that you can rent. Here again, I'm not aware of any site that indexes them, and conditions are often “call or email us”.
If you're truly desperate, many of the largest train stations have a piano. I'm not sure this list is up-to-date; you can look up the amenities of individual stations (it's somewhat buried under “Services & Commerces” then “Divertissement et travail”). Some shopping centres also have a piano. Free, audience included, availability not guaranteeed.
