Thank you for the post Dan. The truth is, it will be very difficult to find chateau that will receive you for a visit without notice. On the bright side, your GF is going to be in Montpellier, France for a year, so you will have future opportunities to visit.
If you're staying at a nice hotel, with a conceriege, you can ask them to help with visits. Else, you can show up and see what happens. Perhaps you will get lucky.
For future visits, read this article, it will help you arrange your next trip to Bordeaux, if you have a little notice. http://www.thewinecellarinsider.com/win ... -tastings/
It's a beautiful place. If you're open to just driving, traveling up the main highways lets you see many of the great chateaux.
If you have other questions, please feel free to ask...