Some of our users have successfully used RuuviTags to measure saunas in even higher temperatures than 85C. Ruuvi CR2477T battery is a must (included in RuuviTag Pro by default). But note that this is not officially recommended by us and will also void the warranty.
We have created a Ruuvi Connector compatible external temperature sensor for prototyping purposes and it is planned to be available relatively soon. It was supposed to be available for sale already by now but we ran into some firmware related issued of which we are going to solve now before opening the sales.
To conclude: Ruuvi is not necessarily the best solution for your use but may work, especially if it is exposed to high temperatures only short amounts of time.