Yes, the Sansovino parking lot is outside of the ZTL area and would be perfect to then get on the tram to go directly to the SMN station. Open space lot, and often has spots open so need to book ahead.
Google maps doesn't place the marker correctly - the address is Via del Sansovino 53, Florence
It is south of the street to the left of the marker google does show - if you look at map at the top of the article here
it is the first blue P sign after the 3 green P signs to the top left of the map.
In addition to agreeing to returning the car if it is a rental and you will no longer need it... I'll add that Bellosguardo is outside of the ZTL area and depending on WHERE you are staying, you might be able to find parking closer or cheaper through your lodging.
Have you asked where you are staying at in case they have parking or have agreement with a garage nearby? If you are staying at a hotel, they might have parking for guests...
While you can't book the Sansovino parking lot ahead of time, it almost always has spaces available.