Pianos and keyboards are items that can get up there in price. They are also more susceptible to damage, especially if individuals rent to children or large groups. I would be willing to bet they are an item that is more rarely offered for usage among rentals.
As someone else said it might be more plausible for you to look into renting one of your own for the duration of your stay. Just be mindful of the potential for noise as well. I love the piano but I could see it getting a bit annoying after so many hrs or days. Just let your host know your intention up front.
If I see anything in my searches I will be sure to pass it along to you. Good luck