Hi
If you reach the RN7, to have taxi brousse to Tana is a little bit difficult. Effectively, as taxi brousse choose mainly passengers who will do the travel Toliara to Tana, all their seats are already full from their departure, only some late taxi brousse will have one or two chairs for people doing the halfway. So I suggest you to take the contact of a taxi brousse cooperative when you are in Toliara, pay a part of the ticket when you are there for a place booking and the day you will be in the RN7, ask him if your place is ok. It should be possible
Steven