If anyone is interested, I just came back from Tunisia and the experience with the Central Bank was great. They absolutely have no issue with people going there and asking for to exchange circulated Tunisian notes to UNC ones. In fact, they also help me get the four notes from the 2011-2013 issue in the best condition they could find.
Next to the Central bank building there is the Money museum, open from Tuesday to Sunday, with a small exhibition about coins (mostly) and notes and a good shop where to buy all Tunisian notes out of circulation.
Fun fact, when I first withdrew Dinars from an ATM at the airport I got 40 pieces of 20TD in UNC condition, :D